Make testbuild target work for multi-locale Android
The testbuild target just provides a single bundle for our platforms to allow a quick test of changes without generating .mar files or localized bundles.
However, that currently breaks generating multi-locale Android .apks with the testbuild target as the step of ignoring non-en-US locales is not done at the packaging step but right from the beginning. The result is that the checkout and assembly of the firefox-locale-bundle is failing ("failing" in the sense that no or potentially the wrong localized files are zipped up; the build of the component itself is not failing).
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