Stop honoring CarrierConfigManager.KEY_HIDE_ENABLE_2G

KEY_HIDE_ENABLE_2G is soft removed in Android API level 35
because it hides a security feature. This patch
introduces simplified logic that ignores the
carrier config value. The new behavior is behind a feature flag.

This patch also includes some cleanup.
1. It removes an unneeded check for null carrier config
2. It removes test logic that set the value of KEY_HIDE_ENABLE_2G
   in places where it had no impact on the test.

Bug: 300248708
Test: atest Enable2gPreferenceControllerTest
Change-Id: I892d115d1ae173d2f3cd69e8f8b97bc5bfa7c67b