Check connection state of media browser

We need to verify that the browser is actually connected before calling
getRoot or getSessionToken, otherwise it will throw an exception (and
crash sysui).

This is not typical but could happen if the service we connected to had
some problem and disconnected soon after we got the onConnected callback

Bug: 156104922
Test: atest com.android.systemui.media

Change-Id: I9b73e2167612a287b527c7bf7bdba1ca2455d55b