Bug 1407366 - Part 1: Rename the LanguagePrompt.jsm to RFPHelper.jsm and changing the place of doing uninitialization. r=johannh

This patch changes the name of LanguagePrompt.jsm to RFPHelper.jsm.
The RFPHelper is going to not only be responsible for the language

--HG--
rename : toolkit/components/resistfingerprinting/LanguagePrompt.jsm => toolkit/components/resistfingerprinting/RFPHelper.jsm
extra : rebase_source : 58e6b8e167dfb01db9115a9b8c929fe533e31592