Toggle widget focus based on hub visibility

When the hub is fully visible, widgets will be focusable and vice versa.
This change also makes the CommunalAppWidgetHostView accessible so that
widget name can be read out by screen reader on focus.
Also labeled the long press action on widgets to hint user on how to
enter the edit mode.

Bug: b/333567994
Test: atest CommunalViewModelTest
Test: manual with TalkBack, Voice, Switch Access
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: Ie566664b819051af6d9c6d8f6ea558ffdd1abcef