Generate chd_merged_sepolicy while building CHD am: 9385a0ee82

Original change: https://android-review.googlesource.com/c/platform/tools/treble/+/2895000

Change-Id: Ic24faa7fa831ebf34a8c99581fbfe330df83c65b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>