Lp1260988 - Decline call voicemail fix

This pull request is a fix for LP: #1260988:

https://bugs.launchpad.net/ubuntu/+source/ofono/+bug/1260988

When a call in incoming state is declined, RIL_REQUEST_HANGUP_WAITING_OR_BACKGROUND should be called, otherwise some operators may not forward the caller to voicemail.

Tested on maguro/r121 using a T-Mobile SIM ( one of the operators that has this problem ).  Also tested that this has no effect on hangup ( local or remote ) of an in-progress call.

Tested on mako/r121 using an ATT SIM w/out voicemail.   In this scenario, the caller gets a busy signal.  As with maguro, tested that this doesn't effect local/remote hangup of an in-progress call.

See the bug for more details.