ART: Move .bss mappings check before .bss init.

Previously the check was done before we compared the dex
file checksums, so an out of date oat file could have
failed the check. Now we're doing the check only if we're
initializing the .bss section for use by an executable oat
file, i.e. after matching the dex file checksums.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 119601889
Change-Id: Ib79d84c5bc2cf0674fc1f76071c589d49663e13c