Add dynamic Preferences indexing

- introduce a new private interface "Indexable".
- refactor Wallpaper and Wi-Fi settings to support this new
interface.
- only index saved/remembered Wi-Fi networks
- also add the capability to remove some data from the Index.

Fragments that want to publish some dynamic indexable data should
implement the "Indexable" interface and provide a static final field
named "INDEX_DATA_PROVIDER" with is the Indexable.IndexDataProvider
interface for providing the data for indexing.

Thru this interface the Index can ask what are the data chuncks to
index.

Change-Id: I31e7212c87b8218efe1a8f3028147cb19e119be6