Implement garbage collection device listener

This CL introduces an instrumentation listener that can run before and
after tests to GC before a memory metric collector. The user can
specify the processes to garbage collect and the amount of time to
wait for memory to stabilize.

Bug: 119048359
Test: atest CollectorDeviceLibTest:android.device.preparers.GarbageCollectionPreparerTest
Change-Id: If77f39aba45b641c5f0157688962db5f8b8be51b