Android: several improvements (#65)

Changes for Android tokens:

- Add missing copyright to some generated Android files.
- Move generated tokens Java package to io.element.android.compound.tokens.generated.
- Make LightColorTokens and DarkColorTokens public so their core tokens can be referenced, but add @CoreColorToken Java annotation, used by a linter to verify it's called on purpose. This can help create temporary color tokens in the Android app before they're added to Compound.
- Fix indentation in some templates so the generated files are properly formatted.
- Create a new CompoundIcons.kt file containing both a helper to reference compound icons in vector format quickly on Android, and get a complete list of the included icons for UI preview and screenshot testing.