Remove the dependency on user setting to enable / disable the lockscreen button support

- Previously, capability to control lockscreen support is provided by a
  setting entrypoint in QR scanner, however now that support is
  deprecated.

Bug: 376247980
Fixes: 376247980
Test: atest QRCodeScannerControllerTest
Flag: EXEMPT bugfix
Change-Id: I35c6066c6a010df9da8464e453e16e0dc3ec9935