Change launch target of Assistant Default App for Auto

This change updates the default app launch target to be the role
selector to make it 1 tap away to switch the default app instead of
2 taps away.

Automotive Settings app already has a top level item for Assistant
& voice, so the current target is still accessible.

Fixes: 183719590
Test: Manual
Change-Id: I59397a0ac6ce9cc1d1e2688ad963d46c757dcfe7