The search API "3 char min" constraint is not suitable for some Asian languages

This is a follow up of the discussion we had regarding vlc-android#2078 (closed)

We restrain the search feature to 3 chars min, which is suitable for languages based on latin alphabet, but isn't really usable for languages using ideograms.

We should probably totally remove this constraint in the Medialibrary and let the ports decide on other constraints (such as wait a few hundred ms between two queries).