Don't process NetworkRequests when there are none.

getTopPriorityRequestPhoneId was making up DEFAULT Id when
there were no requests.  This bogus data was confusing onProcessRequest
and causing it to think we were working with the wrong phone, triggering
a PS detatch.

bug:21988698
Change-Id: Idf9928ac75327509f81c47ecea48d75fbabc89b2