android9: Performance improvements for 720p & hang fix

This PR improves correctness and stays more in line with what the xenial branch provides. Applying these changes results in proper 720p support with adequate performance and visual appearance.

In addition to that, workaround/fix the issue of having the Screencasting object not disconnect itself from Mir properly, resuling in a hang during the destruction of MirConnection (waiting for pending commands to be processed).