Support starting and stopping perfetto tracing sessions

This is important for testing, specifically, to enable starting
in-process tracing sessions.

Test: atest PerfettoTraceTest
Test: atest libtracing_perfetto_tests
Bug: 303199244
Flag: android.os.perfetto_sdk_tracing_v2
Change-Id: Iacfb795f8f60e8fc9dd56666e0022ab7bc7ed121