Address review comments for dispatch audio focus with fade API

- Add IntDef definition, @FocusRequestResult, for the result
  value.
- Remove redundant null pointer exception details from the
  Javadoc, as this is documented by the @NonNull annotation.

Bug: 318430127
Bug: 186905459
API-Coverage-Bug: 308666800
Test: atest -c FadeManagerConfigurationUnitTest
Change-Id: I0e7b8944b75b875317d727d4d62155153259e688