Improve MediaSession isActive documentation

The current documentation was partially misleading and didn't
fully spell out in all places that the main purpose is to
publish the session to the rest of the system (=make it
discoverable via MediaSessionManager.getActiveSessions)

Also update the log statement when the active state changed
to actually log the new active flag, which currently
accidentally logs the playback state (that is logged separately
whenever it changes).

Bug: 417853575
Test: n/a
Flag: EXEMPT log only change
Change-Id: I7658d26e158564b29dbe4cecc90826c8d5b0c8f2