Add falsing for edit mode button

Add a viewmodel for the button. Also, the action to start editing is not
needed anymore as part of TileGrid.

Test: manual, enter edit mode
Test: atest EditModeButtonViewModelTest
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Fixes: 379311646
Change-Id: I6e7a1c864ed2eba1202cee0473816d11f4df96a3