GRANT TRUSTED_OVERLAY as part of start tx

- Make sure the TRUSTED_OVERLAY is only granted after WMShell
  successfully handled the entering PiP transition

Flag: com.android.wm.shell.enable_pip2
Bug: 436270922
Test: atest PinnedStackTests
Change-Id: Ic14f0f14100df13e7065f3cfe8737a2d711c10c9