Removing "focusable" from tile_page in QS

Having focusable set to true was sometimes causing bug in a11y that after toggling dark theme tile, all tiles were read. This happens consistently in some rare device states (not sure what's triggering that state) but never happens otherwise.
Bug occurs when tile_page sometimes receives a11y event TYPE_VIEW_FOCUSED and reads all of its content - that is child tiles.
I don't see any reason why tile_page is focusable as it's not available when going throgh shade with Talkback.

Test: manual on device in that "rare" state: turn on Talkback -> open shade and toggle "Dark theme" tile a few times -> see all tiles are not read after toggling
Fixes: 278181244
Change-Id: I080b3729ba977df89a105a95656d260a662eb7f0