Ensure PIP retains bounds on device rotation.

- When device rotates, ensure that the PIP continues to show in the
  same aspect ratio, snapped to the same logical snap point.
- Move common snapping code to policy so that it can be shared between
  SystemUI and the framework.

Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testPinnedStackInBoundsAfterRotation

Change-Id: I2d9f1a2dc077a55c39acc1ccba982c255e2ff3a4