ui/gfx/android: Simplify Java/Skia bitmap conversions

Replace the explicit memcpy loops for converting between Skia and Java
bitmaps with SkBitmap::{read,write}Pixels.

We also adjust some fields of JavaBitmap to make their data types more
obvious.

Bug: 1144462
Change-Id: Ibc07ec546b38cb5650dc9a1e272e834b3579216a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532250
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Adrian Taylor <adetaylor@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826791}