Improve performance of Enum.valueOf(Class, String)

Enum.valueOf() would loop through all enum values, returning
the final matching one. The common case is that there is one
match: we can loop backwards and return on the first match.

Bug: 32104997
Test: CtsLibcoreTestCases
Change-Id: I11051d28525dd49527e056ecce7f4cdd928cf24d