LatinIME: add a "Download Dictionary" step to the setup wizard

this commit does the following changes.

first we convert the debug class into a real activity, make it launchable
via a custom action.

then we add another step to the Setup Wizard, to download dictionaries and
launch the new activity from it.

this makes sure the user is not confused about the recent changes of LatinIME.
(to remind you, we removed the prebuilt dictionaries and offer them as optional
download inside LatinIME)

Change-Id: If996046b4b664b38dc9de673a74ae592cf6df702
LatinIME: Allow Setup Wizard to be launched by other apps.

This is needed for our NamelessSetupWizard

Change-Id: If493e254fe10dc820912d736fea489cb245c78f9
LatinIME: give setup wizard some intelligence

if we launch the setup wizard and all is setup, it opens LatinIME Settings.
this is ugly, lets better always show the Welcome page and skip other setups
according to what is already configured.

Change-Id: I658f8bb2f4aeed5ac1f8b2d1745ada0d2b94b5f5