Update media2 session references to media3 in framework docs

The androidx.media2 library is deprecated, and the source has been deleted.

Also make the summary fragment of these docs more useful by moving the
warning later in the docs.

Bug: 324249307
Test: Presubmit (javadoc only change)

Change-Id: Ib16ed3343fac426a857128d096d1e04a77f551e7