Move files from .../imagemanager/platform/ios to .../imagemanager/platform/ios/react/renderer/imagemanager

Summary:
To properly setup the Header Search Paths for `use_frameworks!`, we need to move the files that are in the `.../imagemanager/platform/ios` folder into the `.../imagemanager/platform/ios/react/renderer/imagemanager` folder.

This mimic the same folder structure we have also `android`

## Changelog
[iOS][Changed] Moved the files from `.../imagemanager/platform/ios` to `.../imagemanager/platform/ios/react/renderer/imagemanager`

Reviewed By: cortinico

Differential Revision: D43088421

fbshipit-source-id: c3b86a95e67cc1ab9531997bb1bfbf011b7f730f

ReactCommon/react/renderer/imagemanager/BUCK

@@ -13,6 +13,8 @@ load(
     "subdir_glob",
 )
 
+oncall("react_native")
+
 APPLE_COMPILER_FLAGS = get_apple_compiler_flags()
 
 rn_xplat_cxx_library(
@@ -49,7 +51,7 @@ rn_xplat_cxx_library(
     ios_exported_headers = subdir_glob(
         [
             ("", "*.h"),
-            ("platform/ios", "RCTImagePrimitivesConversions.h"),
+            ("platform/ios/react/renderer/imagemanager", "RCTImagePrimitivesConversions.h"),
         ],
         prefix = "react/renderer/imagemanager",
     ),
@@ -61,7 +63,7 @@ rn_xplat_cxx_library(
     ios_headers = subdir_glob(
         [
             ("", "*.h"),
-            ("platform/ios", "**/*.h"),
+            ("platform/ios/react/renderer/imagemanager", "**/*.h"),
         ],
         prefix = "",
     ),