Move `clean-oat-host` to build/core/tasks/art.mk

Since Soong genrules now run in sandboxed mode, they cannot execute
commands like `rm -rf` within `clean-oat-target` to remove physical
directories.

The only solution is to move the `clean-oat-target` commands into
`build/core/tasks`.

Bug: 345133095
Test: m clean-oat; m clean-oat-host
Change-Id: Ifce4d5dc4bdb8e3735784135cf6f2ff143e90fe9