transcoding: Add a new API for client to add UID for a session.

This is needed for MediaProvider to append a UID to an existing
transcoding session when there are multiple client waiting
for the file to be transcoded.

Bug: 171398942
Test: atest  CtsMediaTranscodingTestCases:MediaTranscodeManagerTest
Change-Id: I58e561f7afed16314349764195a65386d1d0b909