Show notifications when default launcher changes

Shows a notification when we have to switch away from Gesture Nav
because the new launcher doesn't support it.

Also shows another notification when previously we had to switch away
from Gesture Nav, but now the new launcher supports it.

Bug: 129532605
Test: Manual test
Change-Id: Ie954dd5dea1b45795f5c0e22ab7cc1cffde4aabe