[android] Update internal documentation about experimental CoreCLR builds for Android

[ivanpovazan]

Description

We should update:

runtime/docs/workflow/building/coreclr/android.md

Line 4 in bfa0276

Through cross compilation, on Linux it is possible to build CoreCLR for arm64 Android.

by adding a description on:

• how to build CoreCLR for Android on supported host platforms and which target architectures are supported
• how to build/run the HelloAndroid sample
• how to build/run a functional test

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Tagging subscribers to 'arch-android': @vitek-karas, @simonrozsival, @steveisok, @akoeplinger
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[ivanpovazan]

/cc: @elinor-fung