Restrict access_mtp permission access to Android/

Ensure only platform signed apps holding the access_mtp permission can
access the sensitive Android/

TODO: On devices without sdcardfs, will also need to give the two new
ext_data_rw and ext_obb_rw gids to these processes. May need to do
same for apps with pass_through mount mode as well

Test: MTP app still has sdcard_rw
Bug: 148555237

Change-Id: I140a2ffec8a5887c7cd25d6976ee19555f2a47c1

services/core/java/com/android/server/StorageManagerService.java

@@ -3941,8 +3941,12 @@ class StorageManagerService extends IStorageManager.Stub
             final boolean hasMtp = mIPackageManager.checkUidPermission(ACCESS_MTP, uid) ==
                     PERMISSION_GRANTED;
             if (mIsFuseEnabled && hasMtp) {
-                // The process hosting the MTP server should be able to write in Android/
-                return Zygote.MOUNT_EXTERNAL_ANDROID_WRITABLE;
+                ApplicationInfo ai = mIPackageManager.getApplicationInfo(packageName,
+                        0, UserHandle.getUserId(uid));
+                if (ai.isSignedWithPlatformKey()) {
+                    // Platform processes hosting the MTP server should be able to write in Android/
+                    return Zygote.MOUNT_EXTERNAL_ANDROID_WRITABLE;
+                }
             }
 
             // Determine if caller is holding runtime permission