GpuMem perfetto producer

To overcome the problem of ftrace gpumem events not emitting anything
during MEC (with apps that do launch-time allocations), this change adds
a perfetto producer to emit initial counter values at the start of a
trace, so that the visualization won't so zero usage.

Test: adb shell perfetto --query | grep gpu.
Test: Take a perfetto trace with android.gpumem enabled
Bug: 157142645
Change-Id: I5c8278754549399ffa51e6e6fc2ca69b51976cb2
Merged-In: I5c8278754549399ffa51e6e6fc2ca69b51976cb2