Add unittests for GpuMemTracer

This change adds some unittests for the GpuMemTracer module to verify
that the producer indeed works.

Bug: 164194338
Test: atest gpuservice_unittest:GpuMemTracerTest
Change-Id: I9f570567310f23261b43c85ab8cdc3ede8717f09
Merged-In: I9f570567310f23261b43c85ab8cdc3ede8717f09
5 files changed