Fix issue where mSwitchingFgAndBgCalls was not cleared after swap.

The mSwitchingFgAndBgCalls flag is used when performing a call swap along
with mCallExpectedToResume to determine if if the call swap was successful.
The existing code in onCallResumed handled the case where the wrong call
has resumed, which indicates that the swap failed.  It did not, however
handle the case where the call which was expected to resume did resume.
In this case the mSwitchingFgAndBgCalls was left set to TRUE.

In the case of this bug, after 2 swaps, a remotely disconnected fg call
would cause the bg call to auto-resume, which it should not.  This is due
to the fact the mSwitchingFgAndBgCalls flag was left TRUE from the last
swap.

Bug: 26115831
Change-Id: I934f2eff54365b5e92b56299a6692857faf617bf