Minor fixes to TypeConverter tests

Fix issues discovered after re-reviewing the code:

- add missing checks;

- prefer to use enums from code generated from XSD or their
  conversions to strings instead of strings, to avoid typos.

Bug: 142480271
Test: atest libmedia_helper_tests
Change-Id: Ia681df71bd9aeca65281d8049aacde87b5d44490
Merged-In: Ia681df71bd9aeca65281d8049aacde87b5d44490
1 file changed