Changes the UX of enable/disable null ciphers to
"Require encryption". The default value is off, making
the default behavior equivalent to the previous version
of the toggle.
Test: atest NullAlgorithmsPreferenceControllerTest
Test: Manual testing in cuttlefish
Bug: 264540492
Change-Id: Iaa0c9c259559a205aacad9fb9af8de6b54782c8d