ImageCapture: add sequence<Point2D> pointsOfInterest and wire for Android

This CL:
- adds a Point2D.idl to describe a coordinate [1] and
- adds a sequence<Point2D> to describe a series of
 points of interest [2]
- adds the correspondents to image_capture.mojom
 (note that I'd rather reuse gfx::mojom::PointF but a
 WebKit presumit prevents me from doing it).
- extends the Java_VideoCapture_setPhotoOptions to
 accept the said list of points of interest
- implements the points of interest for (Auto)Focus,
 limitation: only one such is actually used
 (enforced by assert).

[1] https://w3c.github.io/mediacapture-image/#Point2D
[2] https://w3c.github.io/mediacapture-image/#dom-photosettings-pointsofinterest

BUG=343894, 518807

Review-Url: https://codereview.chromium.org/2257613002
Cr-Commit-Position: refs/heads/master@{#413675}
10 files changed