Set selected widget key for SwitchAccess edit widget action

Widgets aren't highlighted when entering edit mode using Switch
Access, as it doesn't automatically handle long press like TalkBack.
So the selected widget's key needs to be set in the custom action
handler.
Also clear the key if entering edit mode from empty space in hub.

Fixes: 378616348
Flag: EXEMPT bugfix
Test: manually with SwitchAccess enabled
Change-Id: Ie6b362878ad1e5373c4fea32b8b38cd74d4b364a