Clear disconnected connections when detaching.

This bug was happening because there was a disconnected connection
lingering on the background call after it is resumed. Because of
this lingering connection, its state remained on HOLD and did not
return to IDLE.

This caused the phone to get in a bad state, such that when trying
to unhold the conference later on, there foreground call is ACTIVE
and the background call is HOLD, which precludes unhold from working
properly.

This fix for the bug would clear disconnected connections from the
ImsPhoneCall after detaching a connection.

Note: clearDisconnected also updates the state to IDLE, which is why
I deleted that code.

Bug: 18291230
Change-Id: I09b5e274408235a9b6630d40bf3a3161676ffc79