Fix page accounting for multidex

Dexdiag was using the offset into the mapping rather than into the dex
file for determining the types of pages.  This resulted in multidex
files having incorrect accounting.

Bug: 38181622
Test: make test-art-target-gtest-dexdiag_test
Change-Id: I00a4e21db5f57f75642a17e8edaa9c73b8b529b6