Do not rely on /data/apex

With an upcoming feature (mount_before_data), installed APEX will have a
different location (typically a device-mapper block device, e.g.
/dev/block/dm-42). Hence, checking /data/apex path won't work.

Bug: 430161238
Test: BinaryTransparencyHostTest
Flag: com.android.apex.flags.mount_before_data

Change-Id: I540634a4c555b43bc4a5cdb867cfdee02fc2b926