Change a11y behavior of non-current tiles on edit

When editing tiles, when moving a tile using the accessibility action,
we would mark the invalid positions as not important for accessibility.
This would cause the recyclerView to scroll to show these views, moving
the valid views out of reach for the a11y service, but not leaving the
user in a good context.

Instead, have the views report to the a11y service, but set their
content description to indicate that they are in an invalid position.
Apply this also to the "Add tile" action that allows the user to add a
tile to a particular position (as opposed to the end).

Test: manual, using Talkback
Fixes: 319585400
Flag: none

Change-Id: I0abaa2a3dafcb6e99108378a1d6e0ef2267b2660