rilmodem: fix decline call behavior (LP: #1260988)

When a remote call with STATUS_INCOMING is declined,
RIL_REQUEST_HANGUP_WAITING_OR_BACKGROUND should be
called instead of RIL_REQUEST_HANGUP, as some operators
may not forward the caller to voicemail.
Merge pull request #48 from tonyespy/lp1260988

Lp1260988 - Decline call voicemail fix