Honor DISALLOW_CELLULAR_2G User Restriction in Enable2gPreferenceController

When 2g is disallowed by a device admin the 2g toggle in the preference
screen will implement the following behaviors as a result of being
changed to a RestrictedSwitchPreference:
1. become disabled (greyed out)
2. show a summary message explaining that the preference was disabled by
   an admin
3 show a pop up when a user tries to click on the preference informing
  them that the setting is unusable because it was disabled by an admin.

Additionally, the toggle will show as unchecked (off) when admins
disable 2g.

When 2g is re-enabled by a device admin, the preference screen will go back
to its previous state.

Test: atest Enable2gPreferenceControllerTest
Bug: b/248250240
Change-Id: I7af901f2d9f62ebfe884e01724d8eff845c2968e