Do not create new LayerTracing instance within LayerTraceGenerator

LayerDataSource holds a reference to LayerTracing in order to notify
LayerTracing about perfetto events (OnSetup, OnStart, OnFlush, OnStop).

Having multiple instances of LayerTracing complicates things, as it arises
the question of which instance(s) should be referenced and notified
by LayerDataSource.

This commit eliminates the problem at its root, by avoiding to create
an extra LayerTracing instance within LayerTraceGenerator.

Bug: b/293429094
Test: atest transactiontrace_testsuite && atest SurfaceFlinger_test

Change-Id: Id2cf232d1198f9470b476d9a9d9eb86c21246c61