Don't show notifications for locked slots

Locked slots are used for customization. We expect the
client apps to provide custom UI for notifications
and entering/exiting DSU mode.

(Cherry-picked from aosp/2655023)

Bug: 277691885
Test: 1. adb shell am start-activity \
         -n com.android.dynsystem/com.android.dynsystem.VerificationActivity \
         -a android.os.image.action.START_INSTALL \
         --el KEY_USERDATA_SIZE 2147483648 \
         --ez KEY_ENABLE_WHEN_COMPLETED true \
	 --ez KEY_ONE_SHOT false \
	 --es KEY_DSU_SLOT foo.lock
     2. check no dsu notification is shown after reboot

Merged-In: I01de2fd2d5942dd4e5e462b46890ce1f10800768
Change-Id: I01de2fd2d5942dd4e5e462b46890ce1f10800768