Flow type: saveToCameraRoll returns a string

Summary:
Updates the flow typing to return Promise\<string\> instead of Promise\<Object\>. To validate that it actually does return a string, see https://github.com/facebook/react-native/blob/6493a857542c0985470311511c1be80982fcd474/Libraries/CameraRoll/RCTCameraRollManager.m#L98 and https://github.com/facebook/react-native/blob/6493a857542c0985470311511c1be80982fcd474/Libraries/CameraRoll/RCTCameraRollManager.m#L116
Closes facebook/react-native#15631

Differential Revision: D5714842

Pulled By: shergin

fbshipit-source-id: fb141b014c262bc4fb44419515e56bbe0641d8bf

CameraRoll.js

@@ -126,7 +126,7 @@ class CameraRoll {
   /**
    * `CameraRoll.saveImageWithTag()` is deprecated. Use `CameraRoll.saveToCameraRoll()` instead.
    */
-  static saveImageWithTag(tag: string): Promise<Object> {
+  static saveImageWithTag(tag: string): Promise<string> {
     console.warn(
       '`CameraRoll.saveImageWithTag()` is deprecated. Use `CameraRoll.saveToCameraRoll()` instead.',
     );
@@ -150,7 +150,7 @@ class CameraRoll {
   static saveToCameraRoll(
     tag: string,
     type?: 'photo' | 'video',
-  ): Promise<Object> {
+  ): Promise<string> {
     invariant(
       typeof tag === 'string',
       'CameraRoll.saveToCameraRoll must be a valid string.',