[Mono.Android] Fix _GenerateJavaCallableWrappers naming (#15)

[@atsushieno notes][0] that there are *two*
`_GenerateJavaCallableWrappers` targets within
`Mono.Android.targets`, and the latter is likely replacing the
former, preventing `$(OutputPath)mono.android.jar` from being
generated (commit 953e28b; [bug report][1]).

In short, commit a9f69ad screwed up by reusing a target name.

Rename the second `_GenerateJavaCallableWrappers` target to
`_GenerateFrameworkList`, allowing the first
`_GenerateJavaCallableWrappers` target to actually execute.

[0]: https://gitter.im/xamarin/xamarin-android?at=5727fac1df1a01ff18fbdeed
[1]: https://gitter.im/xamarin/xamarin-android?at=5726f22b9344bbcb0f8bb201

src/Mono.Android/Mono.Android.targets

@@ -117,7 +117,7 @@
  Command="jar cf "$(OutputPath)mono.android.jar" -C "$(IntermediateOutputPath)android-$(AndroidApiLevel).jcw\bin" ."
  />
  </Target>
- <Target Name="_GenerateJavaCallableWrappers"
+ <Target Name="_GenerateFrameworkList"
  AfterTargets="CoreBuild"
  Inputs="$(OutputPath)$(AssemblyName).dll"
  Outputs="$(OutputPath)RedistList\FrameworkList.xml">