Forward wallpaper activity results to the caller.

- Added Activity.FLAG_ACTIVITY_FORWARD_RESULT to the intent that
launches the wallpaper activities.
- WallpaperSuggestionActivity also finishes itself after launching
intent.

Bug: 65691577
Test: Added robolectric test. Also verified manually that result code is
being forwarded
Change-Id: I2f8b0fac7a35c38e14c4a733b7552fd2e58dd0db