Cleanup buildMethodSchema and introduce nullable methods

Summary:
## Changes
- Started doing cleanup under `/parsers/flow/modules/methods.js`
- Rename `NativeModuleMethodTypeShape` -> `NativeModulePropertyShape`
- Moved optional property from `FunctionTypeAnnotation` to the `NativeModulePropertyShape`
- Introduced a nullable property inside what was once `FunctionTypeAnnotation`.

Changelog: [Internal]

Reviewed By: PeteTheHeat

Differential Revision: D23146050

fbshipit-source-id: 2fe97bb9c0736242682133e4923131a54bbea54b

packages/react-native-codegen/src/generators/modules/GenerateModuleJavaSpec.js

@@ -13,7 +13,7 @@
 import type {
   FunctionTypeAnnotationParam,
   FunctionTypeAnnotationReturn,
-  NativeModuleMethodTypeShape,
+  NativeModulePropertyShape,
   ObjectTypeAliasTypeShape,
   SchemaType,
 } from '../../CodegenSchema';
@@ -156,7 +156,7 @@ function translateFunctionReturnTypeToJavaType(
 
 // Build special-cased runtime check for getConstants().
 function buildGetConstantsMethod(
-  method: NativeModuleMethodTypeShape,
+  method: NativeModulePropertyShape,
   imports: Set<string>,
 ): string {
   if (