Fixing resume of FG call after failure to merge IMS calls.

The problem was due to a bug in switchWaitingOrHoldingAndActive.  When
we start a swap between FG and BG calls, a flag mSwitchingFgAndBgCalls is
set so that we can detect situations where the swap fails and "unswap" the
calls.

The original switchWaitingOrHoldingAndActive which occurs when dialing out
the new call to add to the conference was failing to toggle
mSwitchingFgAndBgCalls back off because no onCallResume is received (this
is where the flag was turned off in the past).

To fix this I reset the mSwitchingFgAndBgCalls flag when we get an unhold
and do not request a resume.

Bug: 19034876
Change-Id: If9f2360bbf3d7294b856ab021e3a0a8c46f2d928