Create a bug fix flag for the FloatingMaskView.

Need to create a bug-fix flag since feature flags can't reach nextfood in time for V. We do this by renaming
the current flag.

According to https://g3doc.corp.google.com/company/teams/android-core-experiments/trunk-stable/gantry-stages/bug-fix.md?cl=head

After a flag declaration has been merged, it's no longer possible to change its purpose. If the purpose of a flag changes over time, simply rename the flag; this will create a new workflow and invalidate the previous workflow.
In cases where renaming is not possible, please file a bug for the ACE oncall. Even when re-using the name of a flag for a new purpose, please note that all the progress in the pre-existing workflow will be lost; the new bug-fix workflow will start from scratch.

bug: 339850589
Test: Presubmit/locally adb shell device_config put launcher_search com.android.launcher3.private_space_add_floating_mask_view true
Flag: ACONFIG com.android.launcher3.private_space_add_floating_mask_view STAGING
Change-Id: Iffbc6a93c4ef6bbe121e0c8fb83250174f367b9d