Changed ApnSetting mappings to constants

* Changed ApnSetting to use constants instead of hardcoded strings for clarity.
* Made isDataCapable a public api.
* Removed feature flag android.hardware.telephony.data and opted for a resource
config setting instead.

Bug: 146206136
Test: FrameworkBaseTests, FrameworkOptTests
Change-Id: I19ac83f5865e8763cd9414c41d4609f79d0db164