把android P上被谷歌架空的media write权限补回来，解决外置存储卡复制、移动等操作，提示需要root权限的问题。

frameworks/base/data/etc/platform.xml
    <permission name="android.permission.WRITE_MEDIA_STORAGE" >
        <group gid="sdcard_r" />
        <group gid="sdcard_rw" />
        <group gid="sdcard_all" />
        <group gid="media_rw" />
    </permission>

frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java
@Override
public void systemReady() {
...
    StorageManagerInternal.addExternalStoragePolicy(
                new StorageManagerInternal.ExternalStorageMountPolicy() {
        @Override
        public int getMountMode(int uid, String packageName) {
                if (Process.isIsolated(uid)) {
                    return Zygote.MOUNT_EXTERNAL_NONE;
                }
                //  +{ for BUG
                if (checkUidPermission(WRITE_MEDIA_STORAGE, uid) == PERMISSION_GRANTED) {
                    return Zygote.MOUNT_EXTERNAL_DEFAULT;
                }
                //  + } for BUG
                if (checkUidPermission(READ_EXTERNAL_STORAGE, uid) == PERMISSION_DENIED) {
                    return Zygote.MOUNT_EXTERNAL_DEFAULT;
                }
                if (checkUidPermission(WRITE_EXTERNAL_STORAGE, uid) == PERMISSION_DENIED) {
                    return Zygote.MOUNT_EXTERNAL_READ;
                }
                return Zygote.MOUNT_EXTERNAL_WRITE;
            }
    }
...
}