Launch work lock scrim as clear_top not single_top

SINGLE_TOP is only guaranteed to work within the current task stack.
If multiple stacks are inactive in the background (eg. temporarily
switching from freeform mode to fullscreen mode by maximising one
activity) some tasks might get missed out and stay visible.

Test: manual -- start two work activities, one in fullscreen & one in freeform. start a personal
Test: activity in freeform too. switch to the fullscreen work app and lock the profile using
Test: `adb shell service call trust 6 i32 $USER_ID i32 1`
Test: __outcome after change__
Test:   a) locking work profile should bring up the work password prompt in fullscreen
Test:   b) skipping the prompt and switching to the personal freeform app should keep the freeform
Test:      work app hidden by WorkLockActivity
Test: __outcome before change__
Test:   Only (a) happened, for step (b) the work app was still visible
Bug: 31001762
Change-Id: I5979c5b12670a21c7bcd59e222fc518d395e3e29