[Xamarin.Android.Build.Tasks] added filter to proguard commands

[jonathanpeppers]

Context: https://bugzilla.xamarin.com/show_bug.cgi?id=56834

A default multi-dex application will print a warning to the console
such as:

Warning: can't write resource [META-INF/MANIFEST.MF] (Duplicate zip entry [android-support-multidex.jar:META-INF/MANIFEST.MF])

This is due to most jar files containing a META-INF/MANIFEST.MF,
which really just needs to be excluded from the proguard command.
We can do this by adding a filter to the end of each jar file passed
via the -injars command line option.

Changes:

• Added a $(_AndroidProguardInputJarFilter) internal property, which
  defaults to (!META-INF/MANIFEST.MF) so we ignore the most common warning
  by default
• Added a ProguardInputJarFilter property to the <Proguard /> and
  <CreateMultiDexMainDexClassList /> tasks
• Added assertions to multi-dex unit tests making sure we aren't getting
  the warning anymore
• Minor fix to clear the ghprbPullLongDescription environment variable during
  tests, otherwise my PR/commit message causes my new test assertions to fail

[dnfclas]

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[jonpryor]

Should this be a public MSBuild property at all? I'm not sure that this is an extension point that we want to support, or -- alternatively -- I don't fully foresee what we may be signing ourselves up for by exposing this as a public property.

Additionally, I'm not sure about the longevity of ProGuard, given that Google is working on d8/etc. as a replacement.

I'd feel better if this were a private -- _-prefixed -- property, or hardcoded in the task.

[jonathanpeppers]

🤦‍♂️ my two new tests are failing because of this PR's description being passed as an environment variable ghprbPullLongDescription:

ghprbPullLongDescription = Context: https://bugzilla.xamarin.com/show_bug.cgi?id=56834\r\n\r\nA default multi-dex application will print a warning to the console\r\nsuch as:\r\n```\r\nWarning: can't write resource [META-INF/MANIFEST.MF] (Duplicate zip entry [android-support-multidex.jar:META-INF/MANIFEST.MF])\r\n```\r\n\r\nThis is due to most jar files containing a `META-INF/MANIFEST.MF`,\r\nwhich really just needs to be excluded from the proguard command.\r\nWe can do this by adding a filter to the end of each jar file passed\r\nvia the `-injars` command line option.\r\n\r\nChanges:\r\n- Added a `$(_AndroidProguardInputJarFilter)` internal property, which\r\ndefaults to `(!META-INF/MANIFEST.MF)` so we ignore the most common warning\r\nby default\r\n- Added a `ProguardInputJarFilter` property to the `<Proguard />` and\r\n`<CreateMultiDexMainDexClassList />` tasks\r\n- Added assertions to multi-dex unit tests making sure we aren't getting\r\nthe warning anymore

I put Warning: can't write resource [META-INF/MANIFEST.MF] (Duplicate zip entry) in the PR description (as it probably needs to be).

I think the easiest thing is to clear out ghprbPullLongDescription when Xamarin.Android.Build.Tests runs xbuild/msbuild, it is not needed there anyway...