Adding Android Integration

[Danielku15]

Issues

Fixes #532

Proposed changes

Adds the specific platform implementations and toolchain updates for targeting android natively.

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• Existing builds tests pass
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website

[Danielku15]

I finally found out what is causing Kotlin to be so extremely slow with the alphaTab codebase. It seems to be super slow when inferring generic types. One key class there is the PercussionMapper where there is a huge list of DoubleObjectMapEntry objects created without explicitly having generic types. Commenting only the MapEntry creations in the generated Kotlin, reduced the compilation from 3min to 1min on my Surface. Having it uncommented and opening the PercussionMapper makes Android Studio go crazy with high CPU load and memory consumption.

As a local fix we can try to explicitly specifying the generics.
In parallel we should create a standalone project only with the involved classes to report the error more concretely to JetBrains.