Android JNI Refactor

[matteblair]

• Native methods removed from MapController into separate NativeMap class
• NativeMap stores pointer to cpp objects in private field
• NativeMap checks pointer once at initialization, afterwards native methods are called without passing in pointer
• In cpp implementation of NativeMap methods, pointer field is accessed via JNI
• AndroidPlatform and AndroidMap split into separate files
• Functionality shared between AndroidPlatform and AndroidMap moved into JniHelpers
• cpp files renamed to match class names
• Unnecessary fields in MapController converted to local variables