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pkotwicz@ pointed out that res/mipmap is actually an alias for
res/mipmap-mdpi. This resulted in all icons being way too large and
appear with some fidelity issues. In testing I believed these larger
icons to be correct but in reality they should have been smaller. To
fix this we could move the icons into the mipmap-nodpi folder.
However, this prevents them from scaling even when aliased. i.e. if
on an xxhdpi device all we have is an mdpi icon the following
scenario occurs; drawable-*dpi/ has no valid splash_icon.xml so we
fall back to the mipmap-*dpi/app_icon.xml here we find that only the
mdpi icon is available. One would think this then scales up to an
appropriate size on drawable-xxhdpi but instead Android refuses to
scale it as it is in the mipmap-nodpi as the double lookup does not
change the fact it is nodpi. pkotwicz@ then suggested we use Scale
XML resources, these solve the problem but require the minting
server to manually compute the scaling at mint time. Instead I
propose the following solution:

- Restore the state for app_icon.png files prior to CL
  https://chromium-review.googlesource.com/1128256
- Introduce a single splash_icon.xml in drawable-xxxhpdi. This
  will reference mipmap/app_icon and essentially be identical to
  the current behavior of just using the app_icon (this is just a
  placeholder).
- Add a boolean to the metadata of the AndroidManifest.xml
  indicating whether we were able to package larger images on
  the server. (This is so xxhpdi can use these images rather than
  only using the slightly larger xxxhdpi icon).
- Use ApiCompatibilityUtils#getDrawableForDensity(). This gives
  us the image for a specific density or scales the nearest
  available one if there isn't one available. This allows us
  to control which size of image we show. We can then impose the
  following lookup scheme based on the current device density.

Tested on xhdpi, hdpi and xxhdpi running N, O, P

| Current Density | App Icon Density for Splash Icon |
| mdpi            | xhdpi                            |
| hdpi            | xxhdpi                           |
| xhdpi           | xxxhdpi                          |
| xxhdpi          | xxxhdpi or larger if provided    |
| xxxhdpi         | xxxhdpi or larger if provided    |

Bug: 819755
Change-Id: I40fe68375782b29d082193aae0ab1c1af0ae49f2
Reviewed-on: https://chromium-review.googlesource.com/1150297
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581549}