Filter ABI

By default lots of ABIs are built, but we only want to target arm64-v8a

We can ensure we only build arm64-v8a by adding the following to the defaultConfig section of build.gradle for the app module:

ndk
{
    //abiFilters 'x86', 'x86_64', 'armeabi', 'armeabi-v7a', 'arm64-v8a'
    abiFilters 'arm64-v8a'
}