Ensure PipTransitionController initializes in order

- This is the only transition controller that isn't using ShellInit,
  which means that it currently adds itself as a handler too early.

Bug: 238217847
Test: atest PinnedStackTests
Test: atest WMShellFlickerTests:com.android.wm.shell.flicker.pip.AutoEnterPipOnGoToHomeTest
Change-Id: Ia6dd437d52b8e7f249c86ed4b013fcbf04c31265