Require 'exact', 'prefix', or '' for match operation in property_contexts

The previous code would lazily check for 'exact' and accept any other
value as a prefix match.  This should be a tighter check allowing only
'exact', 'prefix', or an empty string for this option.

Test: build fails if an invalid string is used for the match operation
Test: build succeeds normally
Test: `getprop -Z` shows exact vs prefix is differentiated correctly
Change-Id: I21dcb193810d65f468f8960967eabfd261f71e21