Merge branch 'master' into cloud-android-current-release

Key changes:
- Add libnativewindow dependency for Android O (cf. Project Treble).
- Restrict building Subzero to Marshmallow and up (only affects ARM).
- Restrict GLSL globals to be initialized with constant expressions.
- Log an error when unsupported Android pixel formats are used.

Change-Id: I48277f8954c191b50ab2d769375cf511d2458171