[Android] Inject `route` into manifest when specified on the command line in release mode (#191328)

Follow up to https://github.com/flutter/flutter/pull/190222. Routes
`route` through the manifest when used in release mode to follow the
precedent of other Android engine flags as part of the work towards
https://github.com/flutter/flutter/issues/190461.

Separate PR created for clarity; `route` is parsed apart from most other
engine flags.

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