Fix layer tile allocation to be more consistent, less disco

Fix layer tile allocation to be more consistent, less disco

bug:5290559

Two major things:

* Avoid swapping front/back textures before the textures have been blitted to
  avoid race condition where blit fails because it doesn't see a back texture

* Don't allow a tile to acquire its front texture to paint into, as the
  acquisition logic doesn't handle this.

Change-Id: I84b59396ca9faaf3ddc7c75b6f66e4736bf4c3cf