Re-Add a flag to Enable Carrier N1 Mode Control

Add a new flag to control Carrier N1 Mode, so that
it can be defaulted to off. Some OEMs may not have
had sufficient opportunity to update all other
configs in all other software that must correctly
reflect the carrier SA mode support. In the event
that those configs are not updated, enabling this
feature will cause functional regressions. Thus,
disabling this feature by default.

Bug: 328848947
Test: atest GsmCdmaPhoneTest
Test: manually verified on Pixel
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:650660f91426d141a29fff2e9b55003d5e64ad3f)
Merged-In: Ic3d490422ea7973a21a59b5363232f8b8c06f872
Change-Id: Ic3d490422ea7973a21a59b5363232f8b8c06f872