Removed Bridge dependency from image props on Fabric (iOS)

[grahammendick]

The React Native Slider component has image props, for example, trackImage. When first migrating to Fabric, tried to copy the Slider implementation for the image props on BarButton and TabBarItem components. But it uses c++ state which isn’t documented anywhere. So used the ImageLoader module. But it was only a temp solution because it relied on RCTBridge+Private.h. This PR replaces the temp solution with the Slider's c++ implementation.

The ‘Write You Own State’ solution suggested by React Native put me off trying c++ state because it didn’t seem right hand-writing all the codegen classes. But discovered the interfaceOnly flag setting in the NativeComponent.js spec. This still codegens the Props and EventEmitter - classes that change when the spec changes - but not the ComponentDescriptor, State and ShadowNode. These can be handwritten to implement custom State like image props.

Had to update the source_files of the podspec so these hand-written classes were included in the iOS project. But getting these files included on Android is much harder. Even though these files aren’t used on Android, the build fails without them. Had to include them in the Android.mk of the app (user changes).

LOCAL_C_INCLUDES := $(LOCAL_PATH) $(NODE_MODULES_DIR)/navigation-react-native/cpp
LOCAL_SRC_FILES := $(wildcard $(LOCAL_PATH)/*.cpp) $(wildcard $(NODE_MODULES_DIR)/navigation-react-native/cpp/react/renderer/components/navigation-react-native/*.cpp)
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)

Also had to include mapbuffer and imagemanager in LOCAL_SHARED_LIBRARIES

libreact_render_mapbuffer \
libreact_render_imagemanager \

And in MainComponentsRegistry.cpp had to include the hand-written ComponentDescriptors because they’re not part of the codegen’ned descriptors anymore.

#include <react/renderer/components/navigation-react-native/NVBarButtonComponentDescriptor.h>
#include <react/renderer/components/navigation-react-native/NVTabBarItemComponentDescriptor.h>