Identify and fix potential thread safety issues in SplitController

1) Went through SplitController call paths
2) Some methods require locks and were not annotated. Checked the call
   paths for these methods, ensured callers had acquired the lock
   somewhere up the chain, and added GuardedBy to these methods
3) Fixed unsafe area(s):
    A) onStartActivityResult accesses #getContainer, which iterates
       through mTaskContainers, which requires a lock. Updated the
       onStartActivityResult block to use a lock

Bug: 242647030
Test: make androidx.window.extensions RUN_ERROR_PRONE=true
      check warnings
Test: Smoke test with Jetpack WindowManager Demos apk

Change-Id: I1099cf76f88e49c706b65e394afbb82dd4a1bb99