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* Fail if it finds a non-trivial AndroidManifest.xml
* Fail if it finds any .so files
* Fail if it finds any assets
* Support proguard.txt

This also tweaks the naming of the sub-jar targets to give them better
target names (which show up in .jar names).

TBR=bshe
BUG=640836

Review-Url: https://codereview.chromium.org/2309643002
Cr-Commit-Position: refs/heads/master@{#417160}