Change the name of the contexthub service.

Change the name used by the contexthub service. Specifically, this
change switches from using the "ContexthubService.CONTEXTHUB_SERVICE"
constant to using the "Context.CONTEXTHUB_SERVICE" constant- which is in
line with the other Android services.

Merged-In: I18ae73ed0fda2f938e3233670dc52b5692d321ae
Test: GTS tests pass.

Change-Id: I18ae73ed0fda2f938e3233670dc52b5692d321ae