WindowTracing: fix deadlock

Do not write WM data to perfetto in the onStart callback.
Offload the writing to the main thread instead and use
DataSourceParams#postponeStop + stopDone() to synchronize
main and perfetto threads.

Bug: 323165543
Flag: EXEMPT bugfix
Test: atest WindowTracingPerfettoTest
Change-Id: I8182fdcce2ab76d41aa0d0564b6b4a217ad81980