[Mono.Android] Android API-R Developer Preview 4 enumification (#4710)

Enumification for API-R Developer Preview 4 (375e062).

  * Convert constant fields to enums as appropriate.
  * Update methods that take an `int` constant value to use new enums.

There is some "acceptable-breakage" because I renamed some of the enums
that were added in API-R with auto-generated names that had already been
committed in 3ca2a07.

Note that our enums are not "API-aware" so all enumifications are added
for all API levels.  That is, enums added in API-R get added to
`android-21`, `android-22`, etc.…