InCallUI not dismissing after end call is clicked.

In MT InCallScreen, the customMessage dialog
which appears when user tries to reject a call
with message, is not set to null when user
cancels the dialog.
InCallUIActvity is not finished if there is any
pending dialog which is not null and hence this
issue is seen.

Set the dialog variable to null once the dialog
is cancelled by the user.

Bug: 22254547

Change-Id: I86d314461c760387cb5f8099a01da9afe4a79085