Add feature flag in Settings.

User can enable/disable faster emergency phone call feature by switching
this flag from Settings, and the old flag switched by adb command was
replaced by this feature flag.

Test: Manually

Bug: 112168722
Bug: 113539598

Change-Id: Ie38f4657bfd76d8386ce2a075f4ec5d0038f5c3d
Merged-In: Ie38f4657bfd76d8386ce2a075f4ec5d0038f5c3d