Remove "System UI" talkback announcement

"System UI" is incorrectly announced by
Talkback when the switch to managed profile
dialog is shown. This adds an empty
label to the dialog activity so that nothing
is announced, rather than defaulting to the
application label.

Talkback now announces the dialog contents
without starting with "System UI"

Bug: 278858943
Test: Manually tested with talkback enabled
Change-Id: I7728693d66652b30948aebd45f9ab307d6267f17