Create thread for UI operations for selection toolbar

This takes all work off of the main thread. The SurfaceControlViewHost
uses the thread it was created on as the ui thread since it creates it's
ViewRootImpl on this thread as opposed to being created through
ActivityThread.

Test: CtsWidgetTestCases
      CopyContentInSplitGesturalNavPortraitMicrobenchmark
Bug: 421408244
Flag: android.permission.flags.system_selection_toolbar_enabled
Change-Id: I7ceaf4c475e84a98cedacb61e34ca591ffda5008