Give a role to AvailableTileGridCell without stateDesc

Due to b/418803616, in this case, elements outside of the scrolling
container end up being picked up by talkback, but because the text is
not visible, they are described as "Unlabelled".

Giving them a role makes it so that talkback won't pick them.

Fixes: 413271304
Bug: 418803616
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Test: manual, using talkback
Change-Id: I1d65cc235b049481e3f0053a16b60042ebb53c78