Make AudioManager.getSurroundFormats a TestApi

We need to access this API from a TVTS test.

Bug: 187028404
Test: locally run
    m tvts && tvts-tradefed run commandAndExit \
         tvts-all -m TvtsHdmiHostTestCases
Change-Id: I0122ff2a1f285aa01dc76e332a35798e19a07ac7
Merged-In: I0122ff2a1f285aa01dc76e332a35798e19a07ac7