fix WM instance leaks

WindowTracingDataSource used to hold a reference to WindowTracing
that prevented WindowTracing (and in turn WindowManager)
from being garbage collected.  This CL solves the issue replacing
that reference with a WeakReference object.

Flag: android.tracing.perfetto_wm_tracing
Bug: 323165543
Test: atest com.android.server.wm.SystemServicesTestRuleTest#testRule_ranSuccessfully
Change-Id: Ib1c23fb920ae92a192ca870991778c49a148a798