soong: Add `aapt_version_code` default

This appends `--version_code=$(date -u +%Y%m%d)` to aapt flags, which is
useful for flushing some caches upon system updates.

Change-Id: I6575b878f09c1c3138e12abc34d39405f51245e7
Signed-off-by: Noah Anleitner <noah.anleitner@halogenos.org>