Update TelecomManager docs regarding PhoneAccounts

Updates the docs for addNewIncomingCall to mention that if the
PhoneAccountHandle representing a PhoneAccount is not registered or not
enabled, a SecurityException will be thrown (as of b/26864502). Also
notifies the user that PhoneAccounts are stored in DE storage
in registerPhoneAccount.

Bug: 26874536
Bug: 26780305
Change-Id: I2dae932cfa3404d7d5060dad840a75227793603e