Improve checking of multidex dex checksums.

* Fall back to odex file for all multidex entries if the apk is stripped,
  not just for the main multidex entry.
* Verify the number of multidex entries has not changed.
* Improve performance by getting all checksums from the apk in one go
  and cache the results instead of repeatedly opening the apk.
* Stop referring to non-main multidex entries as "secondary" dex files.

Bug: 34604632
Test: added tests to dex_file_test and oat_file_assistant_test
Test: m test-art-host

Change-Id: I58b17ecfbc9165a5bfeffd5281ee21d108f64479