Use /data/misc/apexdata for the list of APEXes

vold_prepare_subdirs should create apexdata directories for each APEX.
Previously, it gets the list by scanning /apex directory. However,
vold/vold_prepare_subdirs run in the bootstrap mount namespace, they can
see only bootstrap apexes in /apex. The reason why it worked was that
unintended side effects of how we managed /apex directory for both mount
namespace.

Instead, since apexdata directories are already populated by init in
/data/misc/apexdata, we can use that directory for the same purpose.

Bug: 295345486
Test: CtsPackageSettingHostTestCases
Change-Id: I453cd59f54ccbb140f73b5e8576b36fa49f9bc59