Backup settings for Assistant invocation

This backs up two more settings:
 * "Use long press home for Assistant" (if user uses 3-button
   navigation)
 * "Use corner-swipe for Assistant" (if user uses gesture navigation)

Since we changed the defaults for these settings between OS generations,
users have been losing their interaction settings when migrating.

Bug: 242160733
Test: atest SettingsBackupTest
      (Temporarily reenabled secureSettingsBackedUpOrDenied)
Change-Id: If4a0fd112f68aebbc343efc9e0d6ccbbe6b89036
Merged-In: If4a0fd112f68aebbc343efc9e0d6ccbbe6b89036