Convert from Handlers to Executors in NotificationLockscreenUserManager

Removal of TestableLooper also means these tests can now run deviceless, so I've made that conversion as well.

Fixes: 315363596
Test: atest NotificationLockscreenUserManagerTest
Test: atest NotificationLockscreenUserManagerMainThreadTest
Test: atest SystemUiRoboTests
Flag: NA
Change-Id: Ie0adfcb9766593b2745791f040d8e3dbbd3e2240