Compatibility fix for apps that construct Notification.Builders early

Some apps in the wild are known to construct a Notification.Builder
from within their Activity constructor, passing said Activity instance
as the Context parameter. However at this point in the Activity's
lifecycle the ContextWrapper's base context has not yet been populated
and most calls to Context methods will fail.

Even though this was technically undefined behavior in the past, as
apps have little to no way to guess at these internal implementation
details it's probably best to preserve old expectations. Add a note
in Notification.Builder for future framework developers and remove the
offending call - the associated field was never used anyway.

ಠ_ಠ

Change-Id: I4cf3df10be14739babc8ae9a969d261a7ecf8beb