Ensure OnScreenHint uses an Activity

While using an appContext WindowManager is useful in certain cases,
for OnScreenHint is MUST have an Activity-based one so that it
can attach views to it. This was changed before but when moved to
AndroidServices layer, regressed. Added explicit doc comment to
ensure we preserve the activity reference.

Bug: 20430202
Change-Id: Id4977bd8913e8bff8848e5241738111dcc10ac74
2 files changed