microdroid: Add support for extra apk files

extra_apk_file is a new label only for APK files passed to microdroid.
microdroid_manager will create directories under /mnt/extra-apk/, and
zipfuse will mount APK block devices to the directories.

Currently only payload can read the files.

Bug: 205224817
Test: manually edit vm config and see APK files mounted
Change-Id: Ie5afb3156f22bb18979ec70904be675e8ff285a7
6 files changed