[Media] Ensure that we update the seekbar visibility whenever its

enabled status changes.

See b/228414210#comment4 for an in-depth explanation.

Fixes: 228414210
Test: manual: Start a media session then update the display size
or font size; verify that the seekbar still shows up.
Test: MediaControlPanelTest

Change-Id: I37ec1149ce24b81ca7f545ee524df68173b3c943