Check media session token type

When checking whether a notification has a media session, make sure that
the value in the extra is the correct type.

In addition, moves this check inside of
Notification#isMediaNotification for consistency - existing usages either
already checked both or will not be impacted, given that valid MediaStyle
notifications are now shown in the media carousel instead of with other
notifications in the shade.

Fixes: 205570941
Test: atest
Test: manual, repro case in bug
Change-Id: I5d0134d2ce81e9e6a2ed16748f84252bf765a1c9