Make builder public for HotwordRejectedResult.

This is the P0 feature we found 10 minutes ago from the client team:
there is currently no way to create HotwordRejectedResult.

Bug: 189189762
CTS-Coverage-Bug: 183425641
Test: atest CtsVoiceInteractionTestCases
Change-Id: I6260a43dda89ba812ac1a6d6b5486145d6777f84