Ideally, Android should also not report availability = true for unsupported URLs.

We don't, but we also don't report false.

I tried rejecting getAvailability() with NotSupportedError but our message advises to try PresentationRequest.start() in this case. I guess returning false is a better strategy then.
Also, the test page doesn't log the availability until I try to start a presentation - then I see both log messages output.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8a9962171b6d2f8e98c9d312c433087425b3f154

commit 8a9962171b6d2f8e98c9d312c433087425b3f154
Author: avayvod <avayvod@chromium.org>
Date: Fri Apr 07 16:27:49 2017

[Android,Cast] Report correctly when devices are not available.

The ChromeMediaRouterDialogController now reports when the source URN is
not supported so that the native side can reject the PresentationRequest
correctly.
CastMediaRouteProvider now reports no sinks available if the source URN
is not supported or if Android MediaRouter's API are not present (like
on JB MR1). For the latter, we now do create ChromeMRP when MediaRouter
is null and check if it's null before using (was already checked
everywhere, except for one spot changed in this CL).

BUG= 706882 
TEST=manual at https://googlechrome.github.io/samples/presentation-api/

Review-Url: https://codereview.chromium.org/2783363002
Cr-Commit-Position: refs/heads/master@{#462887}

[modify] https://crrev.com/8a9962171b6d2f8e98c9d312c433087425b3f154/chrome/android/java/src/org/chromium/chrome/browser/media/router/ChromeMediaRouter.java
[modify] https://crrev.com/8a9962171b6d2f8e98c9d312c433087425b3f154/chrome/android/java/src/org/chromium/chrome/browser/media/router/ChromeMediaRouterDialogController.java
[modify] https://crrev.com/8a9962171b6d2f8e98c9d312c433087425b3f154/chrome/android/java/src/org/chromium/chrome/browser/media/router/cast/CastMediaRouteProvider.java
[modify] https://crrev.com/8a9962171b6d2f8e98c9d312c433087425b3f154/chrome/android/java_sources.gni
[add] https://crrev.com/8a9962171b6d2f8e98c9d312c433087425b3f154/chrome/android/junit/src/org/chromium/chrome/browser/media/router/cast/CastMediaRouteProviderTest.java
[modify] https://crrev.com/8a9962171b6d2f8e98c9d312c433087425b3f154/chrome/browser/media/android/router/media_router_dialog_controller_android.cc
[modify] https://crrev.com/8a9962171b6d2f8e98c9d312c433087425b3f154/chrome/browser/media/android/router/media_router_dialog_controller_android.h