AudioPresentation: Address API review comments

- Added explanation when AudioPresentation.Builder needs to be
  used.
- Add @NonNull to Builder.setLocale parameter.
- Complete parameters docs for Builder's methods.
- Use 'CharSequence' instead of 'String' in Builder.setLabels.
- Mark Builder class as final.

Bug: 125800702
Test: atest CtsMediaTestCases:AudioPresentationTest
Change-Id: Ib2c259439a0fde29612ce75b20f7f318d378d71f