Add crashlytics-ndk support

[Green Appers, Inc.]

I'm getting a SIGSEGV in libflutter.so that flutter_crashlytics doesn't catch.

All that's needed to also catch Android native crashes is to add the gradle dependency com.crashlytics.sdk.android:crashlytics-ndk

And then import com.crashlytics.android.ndk.CrashlyticsNdk, and call Fabric.with(context, Crashlytics(), CrashlyticsNdk())

For now I'll fork flutter_crashlytics and add it myself. Hopefully resulting in fixing whatever bug is causing a native crash.

[Green Appers, Inc.]

Created #63 for discussion.