SAP: Initialize mIntentReceiver to null

* When SAP server quits, it uses mIntentReceiver to determin whether it
  has be registered or not. However, it won't be null if it is
  initialized to an instance of BroadcastReceiver
* This change creates SapServerBroadcastReceiver class and only assign
  mIntentReceiver to its instance in the constructor.

Bug: 66230053
Test: build
Change-Id: I65e587f98ffcc334372c5f8770a499f9ae265358