Create setting key for emergency gesture call number override

This setting will store an emergency number locally if government
provided emergency number (such as 911, 110) are not desired by user,
used by emergency apps such as EmergencyInfo and Personal Safety on
Pixel.

Bug: 172609395
Test: manual/rebuild
Change-Id: I8732c20f0438e521eaec73758242015794c80c3b