Use mInCallActivity if mContext is null.

It seems that attemptFinishActivity can occasionally be invoked when
mContext hasn't been set, via setActivity -> updateActivity ->
attemptFinishActivity, if there are NO_CALLS. In this case, fall back
on mInCallActivity as the context for clearing the display.

Bug: 18728214
Change-Id: I504af612f79e14787741b14b03170805f57c5699