Have patchoat --verify verify symlinks

patchoat --verify now verifies that the vdex and oat symlinks in
/data/dalvik-cache point to the correct vdex and oat files in /system/.
This is to protect attacks that would change the symlink targets.
Refactoring of patchoat code was done too.

Bug: 66697305
Test: make test-art-host-gtest-patchoat_test
Test: remove a symlink and ensure patchoat verification fails
Test: modify a symlink and ensure patchoat verification fails
Change-Id: I18e8f9f6363cf18ad8fa879aeb4d8c7badf679a7