Save PiP activity position on screen for next re-entry into PiP.

- Keep the last known PiP position when the user expands the activity to
  fullscreen until the activity is next hidden or destroyed.

Bug: 36520745
Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testEnterPictureInPictureSavePosition
Test: #testEnterPictureInPictureDiscardSavedPositionOnFinish
Change-Id: I32e2bec8baa000b854981a4492f44c636f083680