Handle EditModeState when starting EditWidgetsActivity.

This changelist makes sure EditModeState is properly updated when
EditWidgetsActivity is started. If canceled (for example, swiping away
the bouncer to dismiss the keyguard), the EditModeState is now cleared.
This changelist also handles preventing multiple invocations to
start the activity.

Bug: 414750775
Flag: EXEMPT bugfix
Test: atest EditWidgetsActivityStarterTest
Change-Id: Iec461450815be2237cc3cfb14d736bc1954c3ffb