Prevent app icons blinking after a user switch.

This CL moves OverviewProxyService from onUserChanged to
onUserChanging, to allow the jank to happen earlier while the user
switch is happening (i.e. while the screen is frozen),

Bug: 275349347
Test: Visual (screen recordings attached to the bug)
Change-Id: I8a5c2f56ff5d94f8c3754acac410f99675a3e1a2