Back up and restore Settings.Secure.IMMERSIVE_MODE_CONFIRMATIONS.

This setting tracks whether the user has seen the "immersive
mode cling" (see ImmersiveModeConfirmation.java):

   Viewing full screen
   To exit, swipe down from the top.

Because this setting has never been backed up, the cling has
reappeared for every user the first time they invoke a
fullscreen activity on a new device.

After this change, each user will only see this dialog once,
ever, as long as they are restoring from a backup.

Manual testing instructions:

    1. invoke a full screen activity so you can acknowledge
       the cling. this will do:
       $ adb shell am start -n android/com.android.internal.app.PlatLogoActivity
       $ adb shell settings get secure immersive_mode_confirmations
       -> confirmed
    2. force a backup (after logging in to a Google account):
       $ adb shell bmgr backupnow com.android.providers.settings
    3. delete the key:
       $ adb shell settings delete secure immersive_mode_confirmations
       $ adb shell settings get secure immersive_mode_confirmations
       -> null
    4. find your backup set token:
       $ adb shell bmgr list sets
       -> abcd1234 : Nexus 5
    5. force a restore:
       $ adb shell bmgr restore abcd1234 com.android.providers.settings
    6. confirm that the key came back:
       $ adb shell settings get secure immersive_mode_confirmations
       -> confirmed

Bug: 328507468
Test: atest SettingsBackupTest
Flag: n/a for this kind of change
Change-Id: I637d8ecfb60ed3986a60c142aa40ad8d38c53e76