[TIMOB-17358] (3_4_X) Android: Update Appcompat assets + scons

This PR contains the following changes:

Update scons to build with Android-21
Update appcompat libraries and assets to run with Android-21
Update android projects to run with Android-21
Update theme.xml 
Fix a crashing issue with the new libraries
Refactor Notification
Add Support for Material Theme
Implement category, priority, visibility properties for notification