Bug 920170 - Move Android prompt service components to their own namespace. r=margaret

--HG--
rename : mobile/android/base/Prompt.java => mobile/android/base/prompts/Prompt.java
rename : mobile/android/base/PromptInput.java => mobile/android/base/prompts/PromptInput.java
rename : mobile/android/base/PromptService.java => mobile/android/base/prompts/PromptService.java