ANDROID: sdcardfs: Define magic value

Define a filesystem magic value to be used for sdcardfs.

Test: HiKey/X15 + Pie + android-mainline,
      and HiKey + AOSP Maser + android-mainline,
      directories under /sdcard created,
      output of mount is right,
      CTS test collecting device infor works

Change-Id: I54daa1452aa6a3ce0401d7d923e0a897f0c26d96
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>