Make CodegenContext reference *android.Context

Previously, it stored an android.Context (no pointer) which while it
worked (because the current contents are themselves pointers) it is
inconsistent with how the rest of the code references it and could
cause weird behavior if additional fields were added to the Context.

Test: m nothing
      m bp2build
Change-Id: I9c70f925dde85e5b15431cc232b0c2eb6371ec6d