Add unit tests for End-aligning short shelf icon container on split shade

Add unit tests for aligning the short shelf's icon container to the end
for split shade on lock screen when the notification minimalism is
enabled.

Bug: 357643925
Flag: com.android.server.notification.notification_minimalism
Test: atest NotificationShelfTest
Change-Id: Iaba2bffd869dcc640256d6aa63dfa4a20b41eea1