Require Android SDK 28 for compilation

[rosen-vladimirov]

Currently applications can be build for Android with SDK 22 or above. However, with latest changes in tns-core-modules, applications cannot be build with SDK lower than 28. The problem is that the we've set min version of Android Support Library to be 28, which cannot be build with anything less than SDK 28. Trying to build such app with SDK 27 for example leads to error for missing resources.
So we need to set min required SDK for building to be 28.
Related issue: NativeScript/NativeScript#6331

[endarova]

We need also to update the setup scripts and the documentation in https://docs.nativescript.org/ to use and suggest Sdk 28 on setup.

[endarova]

Passing compile sdk lower than 28 fails build with tns-android@next #4052

[endarova]

Update Nativescript cli setup scripts to use android sdk 28 #4053