SystemUI: Don't force the UMS dialog above secure lockscreens

A secure lockscreen is supposed to mean that the phone can't be idly
manipulated, and there's no reason why UMS should be an exception.

Yes, the flag here was gated on whether the storage is removable, which
means security of the sdcard is already questionable.  However, that
doesn't tell the full story for devices like DINC, which has an internal
emmc partition that also gets shared on on UMS.

So, let the lockscreen do its job as much as possible, and if a thief
still wants the sdcard, make them do it the hard way...

Change-Id: I247df16a352976943fe3793629e7875fac5153fa
Signed-off-by: Josh Stone <cuviper@gmail.com>