DreamManagerService: Handle missing db value

Should not happen (except for early dogfooders with a specific build),
but handle gracefully as we do elsewhere in dream manager.

To test:
  adb shell sqlite3 \
  /data/data/com.android.providers.settings/databases/settings.db \
  "delete from secure where name = 'screensaver_components'"

Pre-fix workaround:
  adb shell sqlite3 \
  /data/data/com.android.providers.settings/databases/settings.db \
  "insert into secure(name, value) values ('screensaver_components', \
  'com.google.android.deskclock/com.android.deskclock.Screensaver')"

Change-Id: I2cf39608c8dd5011f6a00deece95eb6554da5173