Update "Allow 2G" toggle summary wording.

Update "Allow 2G" toggle summary wording to be more clear regarding
the purpose of the toggle and why it may be disabled.

Bug: b/193693810
Bug: b/196183458
Test: manual & atest -c Enable2gPreferenceControllerTest
Change-Id: Iffef8f4f44105de30392c30ffd44c1d62cd066b8