cleanups in preparation of bigger changes

- fix typo drawForSreenshot misspelled
- get rid of DisplayDeviceBase
- removed unused or unneeded code
- always pass a DisplayDevice to Layer methods that are called
  on a per-display basis (to make it clear that this could be
  called more than once per composition).

Change-Id: Id948b7e09fe5c06db0e42d40d6ed75dd095c7f44