Switch DocumentsUI over to use AppCompat themes.

Switches from Material to AppCompat themes in order to start using
Snackbar.

Also, factor the common bits out of DocumentsNonDialogTheme and
FilesTheme, and make the theme names clearer.

BUG=23518578

Change-Id: I94a00e910469c57315823ae9df33024f2c7360ee
4 files changed