Let ActivityStacks inherit windowing mode from parent

Create ActivityStacks with UNDEFINED override by default.

Added "memory" of previous override mode during transient modes
(split-screen/pip) and restore that instead of hard-coding
fullscreen upon leaving one of those modes.

Added override mode to containers dump output.

Bug: 113900640
Test: go/wm-smoke
      relevant com.android.server.am tests (ActivityStack, TaskRecord,
      ActivityStarter, ActivityRecord, ActivityManagerService).
      Added test to explicitly check override mode.

Change-Id: I4ab51b4b14604b4953fdab6632cb507b8a39d45a