Support dexpreopt against the primary boot image.

Framework and other dex files are used without image.

Test: taimen-userdebug boots when built with
      DEXPREOPT_USE_ART_IMAGE=true
Test: Check logcat for checksum verification failures.
      (Build ART with extra logging in OatFileAssistant.)
Test: Check that bootclasspath-checksums from some prebuilt
      oat files (say input.odex) contain only one image
      checksum followed by dex file checksums with
          grep -az -A1 -E '^bootclasspath-checksums$' <oat-file> | \
          xargs -0 echo | gawk '{print $2}'
Bug: 119800099
Change-Id: I65c2f247656e41f2c37df1ecb9e06af7dabab76e