[r8] fixes to building with gradle

[jonathanpeppers]

Context: #2019

Since 4bb4b2e, our builds have been failing on VSTS on macOS and
Windows. Jenkins is green, however.

macOS

Currently getting a failure building r8 such as:

2018-10-30T20:19:05.2225960Z   * What went wrong:
2018-10-30T20:19:05.2227400Z   Could not determine java version from '11.0.1'.

So I believe the problem here is that Java 11 is in $PATH, and this
is only a scenario on the VSTS macOS build agents.

It turns out I was able to clean up quite a bit in r8.targets:

• We don't need to call SetEnvironmentVariable, I was able to set
  EnvironmentVariables="...;PATH=$(ChromeToolsDirectory)" and things
  still worked fine.
• We should pass --stacktrace to gradle so that we get detailed
  error messages on a failure.

Windows

Currently getting a failure in
Xamarin.Android.LibraryProjectZip-LibBinding.csproj such as:

2018-10-31T14:02:28.8336608Z    .\gradlew assembleDebug --stacktrace --no-daemon
2018-10-31T14:02:40.7122197Z   NDK is missing a "platforms" directory.
2018-10-31T14:02:40.7122937Z   If you are using NDK, verify the ndk.dir is set to a valid NDK directory.  It is currently set to C:\Users\dlab14\android-toolchain\sdk\ndk-bundle.
2018-10-31T14:02:40.7124870Z   If you are not using NDK, unset the NDK variable from ANDROID_NDK_HOME or local.properties to remove this warning.

It doesn't really make sense to me why this started happening with
d8/r8 support... The ~\android-toolchain\sdk\ndk-bundle path seems
completely wrong.

However, it looks like we should be setting
ANDROID_NDK_HOME=$(AndroidNdkDirectory). This is the only other
place we are calling gradle, so it is somehow related to the d8/r8
changes.

[jonathanpeppers]

Will rework this to fix Windows first--as I have figured out what is happening there.