Cannot create APKs for 64-bit x86 and arm architectures

There's no way to produce an APK for x86_64 or arm64 architectures.

1. Install https://github.com/couchbaselabs/Couchbase-Lite-PhoneGap-Plugin
2. Build for android using `--gradleArg=-PcdvBuildMultipleApks=true --gradleArg=-PcdvBuildArch=x86`
3. The resulting APK includes only lib/x86 .so files

$ unzip -t build/outputs/apk/test-x86-debug.apk

To enable building APKs for 64-bit architectures cdvBuildArch should support two additional values x86_64 and arm64.