Explicitly madvise boot image oat files

This restores a subset of previous behavior for madvising all oat
files, specifically for boot image oat files used for execution.
This can greatly improve ZygoteInit performance for class loading.

Bug: 239467545
Test: m + inspect madvise logs
Change-Id: I31deb6b0e57f4e3a5497eda944031280759da999