Improve removing PROPERTY_IS_DOWNGRADED_CONFERENCE from connection

For some operators network, the address of SRVCC conference is not
"Conference call" and is an unidentifiable character as phone number.
SRVCC conference connenction should not remove
PROPERTY_IS_DOWNGRADED_CONFERENCE, so only remove it from connection if
its address is an identifiable phone number.

Test: manual
Test: atest TelephonyConferenceControllerTest
Bug: 233552226

Change-Id: I7df7295facbe16a3b3edf4ca89074b1f95cb36a3
1 file changed