BinaryTransparencyService: Fix getOriginalApexPreinstalledLocation

The existing `getOriginalApexPreinstalledLocation` method makes use
of heuristics to determine the filename of originally APEXs
preinstalled on the /system/apex directory.

This change fixes the method to perform prefix comparison instead
of relying on previous heuristics, and thus making this method
more robust overall against package name mismatch, and provides
clear error signals now.

Bug: 259349011
Test: atest BinaryTransparencyServiceTest
Change-Id: Iff6d1041c71e10522f744b9569f5827bb14ed238