OMS: Introduce default overlays

Default overlays are applied at first boot. They are specified by
package name in system property ro.boot.vendor.overlay.theme as a
semicolon separated list.

Eg:

ro.boot.vendor.overlay=com.android.vendor.overlay;com.android.vendor.overlay_two

Test: manual (remove /data/system/overlays.xml and reboot)
Change-Id: I8a7443a9059ccabfe1a63b075b95f1b2b6db7dbd