Support including apk inside an apex

New property "apps" is added to APEX soong module. This property
configure a list of APKs to package inside APEX payload. For each
android_app module package it's APK into /app/$(android_app module Name)
directory.

Bug: 139906763
Test: m -j
Change-Id: Ic6655f211951af08097c76e157a07ebc52425718