FirebaseAndroid: fix & remove nullability annotations

The nullability annotations seem to cause warnings due to
`-Wnullability-completeness`. Correctly annotating the types as
`_Nullable JavaVM * _Nonnull` seems to not work across Swift and C/C++.
Rather than try to force this, simply avoid the nullability annotations.

Clear up the method registration which was never being invoked due to an
early return in the success case.

Sources/FirebaseAndroid/include/FirebaseAndroid.h

@@ -9,9 +9,9 @@
 extern "C" {
 #endif
 
-jobject _Nullable SwiftFirebase_GetActivity(void);
-JNIEnv * _Nullable SwiftFirebase_GetJavaEnvironment(void);
-JavaVM * _Nullable SwiftFirebase_GetJVM(void);
+jobject SwiftFirebase_GetActivity(void);
+JNIEnv *SwiftFirebase_GetJavaEnvironment(void);
+JavaVM *SwiftFirebase_GetJVM(void);
 
 #if defined(__cplusplus)
 }

Sources/FirebaseAndroid/jni.c

@@ -53,10 +53,10 @@ FIREBASE_ANDROID_ABI
 jint JNI_OnLoad(JavaVM *vm, void *reserved)
 {
   g_VM = vm;
-  if ((*g_VM)->GetEnv(g_VM, (void **)&g_Env, JNI_VERSION_1_6) == JNI_OK)
-    return JNI_VERSION_1_6;
+  if ((*g_VM)->GetEnv(g_VM, (void **)&g_Env, JNI_VERSION_1_6) != JNI_OK)
+    return -1;
   RegisterNativeMethods(g_Env);
-  return -1;
+  return JNI_VERSION_1_6;
 }
 
 FIREBASE_ANDROID_ABI