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This actually hooks up (i.e. passes from invoker) several variables
controlling apk creation (version name, version code,
use_relocation_packer, use_chromium_linker, unzip_libraries_from_apk).

Adds support for overriding some configuration in the private repo.

Adds support for public_configs in grit_rule.gni.

Adds some junit targets (these are built as android libraries currently
instead of host libraries, but that just means we do a bit extra work
that we don't really need).

BUG=359249

Review URL: https://codereview.chromium.org/659703002

Cr-Commit-Position: refs/heads/master@{#301039}