Mount private volume hook after prepareUserData

As CE and DE data directory may not be ready when volume is mounted
(like a new volume). We try to mount data mirror after user data is
prepared. It will skip mounting data mirror if it's already mounted.

Bug: 149027213
Test: After reverting ag/10156695 and aosp/1218838, can see virtual disk
is mounted correctly in adb shell mount, and test app works and mounted
correctly after pm move-package.

Change-Id: If433914a93f7e6154f31715869f52b387157b2dd