Fix preinstalled path of APEX

When package name of one APEX is a prefix of another, the binary
transparency service may return the wrong preinstalled path and
measurement. For example, preinstall path of com.android.media
should not be /system/apex/com.android.media.swcodec.capex.

Besides fixing the issue, the implementation is switched from iterating
the /system/apex directory, to querying apexservice. This may give us
vendor APEX for free, though I haven't been able to confirm yet.

Bug: 262259909
Test: Diff `adb shell cmd transparency get apex_info -v` before and
      after. Saw the only change is the corrected pre-install path.
Change-Id: Ib7cc87dee17091c379c679cd5bece49120077f54