Memtrack HAL: Report global total GPU-private memory

Update memtrack hal documentation to allow reporting the global
total GPU-private memory from the getMemory() API.

Specify how to handle unsupported memtrack operations.

Bug: 193226716
Bug: 193465681
Bug: 192621117
Test: N/A
Change-Id: I6fcebd16fafdc34cc662137784e86750ee907eee
1 file changed