RegisterWithPerfetto only from SystemUI main process

Subprocesses were getting frozen, and this caused perfetto to wait for
them indefinetely (hitting a timeout) when tracing for performance
monitoring began

Fix: 353317355
Test: Boot tests
Flag: NONE fix for perf regression
Change-Id: Icda2bb82a97a897aed5269f5336ea0393f00d9f7