Modify android build.gn to always include debug symbols in release builds

[gmackall]

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[gmackall]

Closing for the time being. I mostly put up this PR because someone had suggested that the bucket that we host engine builds in (which are then available for download by the framework's groovy code) also hosted builds from PR's. But that ended up not being the case.

Anyways, we should still do something similar to this, but not exactly this, and I don't have time to work on this at the moment.