Disable fallback when comparison result is different

Follow-up from ag/18452103, where we add fallback code that runs
with the importer to make sure they are identical.
When the result is different, we'll take the result from fallback
code to minimize the rollout risk. However, since the OEMs might
change the importer implementation. The fallback code would no
longer valid and that makes OEM modified code not working. Hence
the fallback code must be disabled before release.

This change keeps comparison enabled for all cases to keep getting
signals from beta users. And will switch it to read overlay value
for OEM to debug their solution.

Ignore-AOSP-First: Parent CLs are not in aosp yet
Test: 1. NetworkStatsServiceTest
      2. Test all datasets with script
Bug: 233752318
Change-Id: I869ff05297149bde6e13a204bd8c5a4fece75de0