LatinIME: Allow importing external dictionaries

The code is already present but hidden in LatinIME debug settings
Enable the feature and improve the dialog message a bit

The user simply needs to have a .dict dictionary (one or more) in
the /Download folder of his storage.

Change-Id: I93f4be4c41c02ba6306c6a29cf4d1178dd9837b3
Merge remote-tracking branch 'cm/cm-11.0' into HEAD