[Media] In MediaResumeListener, always disconnect the mediaBrowser when

it changes.

In some places in MediaResumeListener, like #getResumeAction, we were
re-setting the value of `mediaBrowser` without disconnecting the old one
(if it existed), so this might be causing the issue where SysUI is still
bound to an old service.

I haven't been able to repro b/225403871 yet so I'm not sure if this
does fix it but I think this is a good change regardless, and once it's in
some builds we can see if it helps.

Bug: 225403871
Test: MediaResumeListenerTest
Test: manual: verified different media cases still work
Change-Id: Ia123fcdce537ca845c62aa29958426d48e563cf7