[build] fixes for failing Windows build

Context: #2019
Context: http://build.devdiv.io/2168276
Context: http://build.devdiv.io/2169681

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

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)`.

After that change, we got a new error:

    2018-10-31T17:19:30.1558223Z   Checking the license for package Android SDK Build-Tools 25.0.2 in C:\Users\dlab14\android-toolchain\sdk\licenses
    2018-10-31T17:19:30.1562674Z EXEC : warning : License for package Android SDK Build-Tools 25.0.2 not accepted. [E:\A\_work\14\s\tests\CodeGen-Binding\Xamarin.Android.LibraryProjectZip-LibBinding\Xamarin.Android.LibraryProjectZip-LibBinding.csproj]

So this is the actual problem!

In fa57aa8, I changed `android-toolchain` to not run
`<AcceptAndroidSdkLicense/>` all the time. That was actually making
this `.\gradlew` command successfully install Android SDK Build-Tools
25.0.2 and use it without a license prompt. We started seeing the
problem since 4bb4b2e, because it caused a new
`~\android-toolchain\sdk` directory to get setup.

The fix here is to just use Build-Tools 28.0.0, since that is the
version we boostrap and install during our build.

Other changes:
- Added `--stacktrace` to the gradle calls in `r8.targets`, so we get
  better error messages. It also matches the gradle command in
  `Xamarin.Android.LibraryProjectZip-LibBinding.csproj`.

Author: jonathanpeppers

src/r8/r8.targets

@@ -5,7 +5,7 @@
 
   <PropertyGroup>
     <!--NOTE: we don't want a gradle daemon locking directories on Windows-->
-    <_GradleArgs>--no-daemon</_GradleArgs>
+    <_GradleArgs>--stacktrace --no-daemon</_GradleArgs>
     <BuildDependsOn>
       ResolveReferences;
       _BuildR8;

tests/CodeGen-Binding/Xamarin.Android.LibraryProjectZip-LibBinding/Xamarin.Android.LibraryProjectZip-LibBinding.targets

@@ -46,7 +46,7 @@
       <_Jdk9Modules Condition="$(_JdkVersion.StartsWith ('9'))">JAVA_OPTS="--add-modules java.xml.bind"</_Jdk9Modules>
     </PropertyGroup>
     <Exec
-        EnvironmentVariables="ANDROID_HOME=$(AndroidSdkDirectory);JAVA_HOME=$(JavaSdkDirectory)"
+        EnvironmentVariables="ANDROID_HOME=$(AndroidSdkDirectory);ANDROID_NDK_HOME=$(AndroidNdkDirectory);JAVA_HOME=$(JavaSdkDirectory)"
         Command="$(_Jdk9Modules) .\gradlew assembleDebug --stacktrace --no-daemon"
         WorkingDirectory="$(MSBuildThisFileDirectory)java\JavaLib"
     />

tests/CodeGen-Binding/Xamarin.Android.LibraryProjectZip-LibBinding/java/JavaLib/library/build.gradle

@@ -2,7 +2,7 @@ apply plugin: 'com.android.library'
 
 android {
     compileSdkVersion 25
-    buildToolsVersion "25.0.2"
+    buildToolsVersion "28.0.0"
 
     defaultConfig {
         minSdkVersion 19