Don't obtain ril request if request not supported

* Outstanding ril requests detect error conditions and
  recieve an onError callback that sends the consumer's
  message with an error code.
* On REQUEST_NOT_SUPPORTED cases, the consumer messsage is
  sent back with error code.
* Because of this redundancy, RilRequest.obtain should not
  be called in the REQUEST_NOT_SUPPORTED case, otherwise,
  the message can be sent twice causing a crash.

Test: telephony tests
Bug: 176924948
Change-Id: I69106ef005485205db3b4d189523c5daac4e430f