IMS: Fix Conference URI button visibility issue.

Don't hide and clear conference dialpad explicitly when
press back key. The same will be updated when show/hide
conference dialpad as part of Activity's UI refresh.
Fix edit before call issue when last used dialpad fragment
is conference URI dialpad.

Change-Id: I3a7195794c95ce1877e13500b05877c791bd484a
CRs-Fixed: 1083364