Add API to get GPU private memory

Remove getGpuDmabufUsageKb since the kernel no longer
supports the sysfs nodes that this depends on.

Add getGpuPrivateMemoryKb which uses the memtrack hal
to report the system-wide GPU private memory.

Bug: 193226716
Bug: 193465681
Bug: 192621117
Test: dumpsys meminfo
Change-Id: Ib90505ec184339d5acbcd42c67dad3ad65dbd933