Jelly: Define base class for Activities containing WebViewExt

WebViewExt and its supporting classes often use the given Context
object as a MainActivity object. This is currently always true, but
casting objects blindly can easily cause issues. Define an abstract
class for the activities intended to be used as container of
WebViewExt objects and use that as base activity for MainActivity.
This solves the Context<->MainActivity ambiguity and allows to
safely call the methods of MainActivity.

Change-Id: Ifc1e29950af3390a512dc8ff25acbe356798b5bf