[Mono.Android] Support API-O Rebuilds (dotnet#579)

*Rebuilding* API-O (571546a) results in a build failure:

	# First build is fine:
	$ make framework-assemblies API_LEVELS=26 CONFIGURATIONS=Debug

	# Second+ builds are not:
	$ make framework-assemblies API_LEVELS=26 CONFIGURATIONS=Debug
	...
	obj/Debug/android-26/mcw/Android.Content.IComponentCallbacks.cs(23,17): error CS0101: The namespace 'Android.Content' already contains a definition for 'IComponentCallbacksInvoker'
	...
		 170582 Error(s)

The cause of the build error is that
`Android.Content.IComponentCallbacks.cs` -- among many other files --
*is* specified twice (!):

* obj/Debug/android-26/mcw//Android.Content.IComponentCallbacks.cs
* full/path/to/…/xamarin-android/src/Mono.Android/obj/Debug/android-26/mcw//Android.Content.IComponentCallbacks.cs

This in turn is related to 98880bd: There are two "environments" in
which `Mono.Android.csproj` is built:

* A "clean" environment
* A previously built environment

In the clean environment, `Mono.Android.projitems` -- created by
`generator.exe` in the `_GenerateBinding` target -- does not exist.
Consequently, after `generator.exe` is run, all of the generated
`$(IntermediateOutputPath)\**\*.cs` files are added to `@(Compile)` so
that the compiler will "see" all of the generated files and things
will build.

In the previously built environment, `Mono.Android.projitems` *does*
exist, and is `<Import/>`ed. The `_GenerateBinding` target is
*skipped*, "preventing" the `@(Compile)` group from being updated to
contain the generated files -- which is desirable, because those files
are already listed in `Mono.Android.projitems`, and including them
would result in "duplicating" the files provided to the compiler.

Note that the `_GenerateBinding` target depends on the
`_GenerateApiDescription` target, so that when
`_GenerateApiDescription` creates an `api.xml` file, then
`_GenerateBinding` will *re-execute*.

This is where API-O rebuilds go wrong:

 1. This is a previously built environment, so
    `Mono.Android.projitems` (1) exists, and (2) is `<Import/>`ed.
 2. The `_GenerateApiDescription` always executes (!).
 3. (2) results in `_GenerateBinding` always executing, causing the
    `@(Compile)` group to be updated, resulting filenames being
    provided to the compiler *twice*.

(2) is where things go bad:

	Target _GenerateApiDescription needs to be built as input file 'Profiles/api-26.xml.in' does not exist.

Specifically, `_GenerateApiDescription` listed as an `Input` the file
`Profiles\api-$(AndroidApiLevel).xml.in`. However, commit 571546a
named the API-O binding `Profiles\api-O.xml.in`, meaning
`Profiles\api-26.xml.in` does not exist, *and won't exist* (until
API-26 is actually released as "stable" by Google, at some unknown
point in the future).

Which is why API-O rebuilds fail. :-)

The fix? Don't use `$(AndroidApiLevel)`, use `$(AndroidPlatformId)`.
For API-O, `$(AndroidPlatformId)` will have the value "O", allowing
`Profiles\api-O.xml.in` to be found, preventing
`_GenerateApiDescription` from unnecessarily executing.

---

This scenario *does* raise a potential (unresolved) workflow problem:
when:

 1. `Mono.Android.csproj` is built, and
 2. `Profiles\api-*.xml.in` is modified, and
 3. `Mono.Android.csproj` is built again,

then everything will break in the same manner as described here:
`Mono.Android.projitems` will exist, thus adding generated files to
`@(Compile)`, then the `_GenerateApiDescription` & `_GenerateBinding`
targets will execute, re-adding files to `@(Compile)`, and things will
break.

A subsequent rebuild will "fix" the error, so long as a
`Profiles\api-*.xml.in` file isn't modified, but this will be annoying
to anyone who encounters it.

Author: jonpryor

src/Mono.Android/Mono.Android.targets

@@ -47,14 +47,14 @@
   </ItemGroup>
   <Target Name="_GenerateApiDescription"
       BeforeTargets="_GenerateBinding"
-      Inputs="Profiles\api-$(AndroidApiLevel).xml.in"
+      Inputs="Profiles\api-$(AndroidPlatformId).xml.in"
       Outputs="$(IntermediateOutputPath)mcw\api.xml">
     <MakeDir Directories="$(IntermediateOutputPath)mcw" />
     <PropertyGroup>
       <ApiMerge>..\..\bin\Build$(Configuration)\api-merge.exe</ApiMerge>
       <_Profiles>@(_AndroidProfile->'%(Identity)', ' ')</_Profiles>
       <_Glob>-s 'Profiles\api-*.xml.in'</_Glob>
-      <_LastProfile>--last-description=Profiles\api-$(AndroidApiLevel).xml.in</_LastProfile>
+      <_LastProfile>--last-description=Profiles\api-$(AndroidPlatformId).xml.in</_LastProfile>
       <_Out>-o "$(IntermediateOutputPath)mcw\api.xml"</_Out>
     </PropertyGroup>
     <Exec