Add the addServiceAnnouncement API for EMBMS

Add addServiceAnnouncement to MbmsDownloadSession, allowing apps to add
service announcements from group call servers and receive information
about available files via the MbmsDownloadSession API surface.

Fixes: 144368478
Test: CTS
Change-Id: I760400f56219879fe71d0115042a3be2f83cef90