Prevent floating mMediaBrowser reference

When switching to a new browser, only disconnect and change to the new
browser once connection succeeds. This will allow the old browser to be
valid and targetable until a new browser connects.

Bug: 62328135
Test: Switch between browsable players on VW carkit
Change-Id: I5701138f9adf4e8c6e08629d86583dbe87290542
(cherry picked from commit 24ff0081)