Remove label for UdfpsTouchOverlay

It shouldn't be focusable or else it'll be focused
whenenver the user has UDFPS running on the lockscreen
which detracts from the other clickable elements
on the lockscreen.

Flag: ACONFIG com.android.systemui.device_entry_udfps_refactor DEVELOPMENT
Test: manually enable a11y and enroll UDFPS with refactor enabled.
observe the entire udfps touch overlay doesn't get selected/focused
Bug: 305234447

Change-Id: I3e841b030c53b8cfb62baf443279a5401c23bd05