Do not use /data/apex/active

With mount_before_data, installed apexes will be mounted from virtual
block devices (device-mapper devices to be precise). Hence the paths
would look like /dev/block/dm-7, not /data/apex/active/...

Hence, checking if the APEX path is not from the preinstalled location
(e.g. /vendor/apex) is fixed in this CL.

For the same reason, deleting those installed APEX files in cleanUp() is
fixed. In fact, these files don't need to be deleted because once the
preinstalled apexes are deleted, those dangling data apexes are deleted
by apexd after reboot.

Bug: 430161238
Flag: com.android.apex.flags.mount_before_data
Test: StagedRollbackTest, StagedInstallInternalTest
Change-Id: I7f0bdc4ffa6f9179d83f30846eef752ab2d163b4