NSNumber arguments must now be nonnull

Summary:
The bridge implementation on React Android does not currently support boxed numeric/boolean types (the equivalent of NSNumber arguments on iOS), nor does Java support Objective-C's nil messaging system that transparently casts nil to zero, false, etc for primitive types.

To avoid platform incompatibilities, we now treat all primitive arguments as non-nullable rather than silently converting NSNull -> nil -> 0/false.

We also now enforce that NSNumber * objects must be explicitly marked as `nonnull` (this restriction may be lifted in future if/when Android supports boxed numbers).

Other object types are still assumed to be nullable unless specifically annotated with `nonnull`.

React/Base/RCTModuleMethod.h

@@ -26,6 +26,7 @@ typedef NS_ENUM(NSUInteger, RCTNullability) {
 
 @property (nonatomic, copy, readonly) NSString *type;
 @property (nonatomic, readonly) RCTNullability nullability;
+@property (nonatomic, readonly) BOOL unused;
 
 @end