Convert vold_prepare_subdirs to C++

Minimize overhead in boot by replacing shell script invoked multiple
times with a C++ program invoked once.

Bug: 67901036
Test: create user, run adb shell ls -laZ /data/misc_ce/10; delete user
    and check logs.
Change-Id: I886cfd6505cca1f5b5902f2071e13f48e612214d
6 files changed