Add keep clear rects API

This allows the app to set views as keep-clear areas, which the system
will avoid laying out floating windows over, like Pip or Bubbles.

Bug: 209577354
Bug: 209578131
Test: atest CtsWindowManagerDeviceTestCases:KeepClearRectTests
Change-Id: I02f72376b709c0534bd678b7ce66cd95cab41832