Load the notifications of the target user before unfreezing the screen

This CL moves NotificationLockscreenUserManagerImpl from onUserChanged
to onUserChanging, in order to hide the previous user's notifications
and load the target user's notifications before the user switch is
complete.

Flag: LOAD_NOTIFICATIONS_BEFORE_THE_USER_SWITCH_IS_COMPLETE
Bug: 275042474
Test: atest NotificationLockscreenUserManagerTest
Change-Id: I2e8498668ee34c726abe21aef7bade5f3222508b
2 files changed