Setup Android/, Android/data and Android/obb dirs correctly.

Normally sdcardfs takes care of setting up these directories on-demand,
for example when an app requests its private data directory to be
created. On devices without sdcardfs however, we ourselves need to make
sure to setup the UID/GID of these directories correctly.

Introduce a new PrepareAndroidDirs() function which sets the dirs up
correctly. On devices without sdcardfs, that means:

Path              UID         GID         mode
/Android          media_rw    media_rw     771
/Android/data     media_rw    ext_data_rw  771
/Android/obb      media_rw    ext_obb_rw   771

Bug: 146419093
Test: wipe Android/, reboot, with and without sdcardfs, verify
      contents

Change-Id: I3a879089422c7fc449b6a3e6f1c4b386b86687a4