Add a Compat Flag for Throwing For Null Telephony

In order to prevent a sea of random crashes as part
of migrating to throwing behavior for callback APIs that
hit null ITelephony and null IOns objects, this change
re-introduces backwards-compatible behavior for APIs that
are being "upgraded" to throwing behavior.

Bug: 182185642
Test: atest CtsTelephonyTestCases
Change-Id: I492ba6a79e74350986afbd6ab5cc5e7b05b8cd35