Update AndroidSdkBuildToolsVersion

[jonpryor]

For sanity/consistency, $(AndroidSdkBuildToolsVersion), $(AndroidSdkPlatformToolsVersion), and $(AndroidSdkToolsVersion) should be updated to be consistent with the latest supported $(TargetFrameworkVersion). As we now support v9.0 (API-28), these properties should be updated to use the 28 versions of these packages, e.g. build-tools 28.0.3:

https://developer.android.com/studio/releases/build-tools

Addtionally, we should update the HowToAddNewApiLevel.md docs should be updated to mention that these properties should be updated when a new API level is added.

[awattar]

In which Xamarin.Android version it will land? 9.2?