Add ability to disable GL draws.

This feature is intended for use in headless test environments,
where disabling drawing has minimal compatibility impact and can
save significant resource usage when running UI tests.

This feature is controlled via a new system property
'debug.hwui.drawing_enabled', and a HardwareRenderer public API that can
override the property value on a per-process basis.

Bug: 188559292
Test: boot emulator, atest CtsGraphicsTestCases
Change-Id: I93c21e680382e03342f235dbf58bd7a5c8a6f767