Manually add optimization flag for Android Release builds. (#12390)
With recent versions of NDK (since 23), the `-O` optimization level compile flag is not being passed when building in the "Release" configuration.
More details here: https://github.com/android/ndk/issues/1740
Our "Release" Android builds have been built without the optimization flag since we upgraded from NDK 21.
This change is a workaround to manually add `-O3` for "Release" Android builds.