WebView: document requirement for Activity Context

No change to logic, documentation change only.

WebView requires an Activity Context in order to work properly (several
features depend on this).

This CL documents this in each WebView constructor by specifying
"Activity Context" for the Context param. This CL also adds a note in
WebView(Context) to explain the importance of an Activity Context.

Fixes: 67945912
Test: make -j40 docs (manually verify it looks good)
Change-Id: I6f84093f7ce39f6bd924fd6e27737f047348d8a4