odrefresh: ensure dalvik-cache exists for cache-info.xml file

Ensure ART module apexdata/dalvik-cache directory exist before writing
cache-info.xml.

Minor comment clarifications.

Bug: 160683548
Test: manual, observe updated module only recompiles on initial reboot.
Change-Id: I51e5f0bc35bc6de5ae3bfb4ce7a6ab5e0dd4fe29