Move update-payload-key.pub.pem into a module.

This allows update_engine listing the file as a required module,
regardless of the value in AB_OTA_UPDATER.

Bug: 130516531
Test: Build aosp_arm64-userdebug w/o setting AB_OTA_UPDATER. Check that
      /system/etc/update_engine/update-payload-key.pub.pem is available.
Change-Id: Ied041aca750e7260402ae8dbf65ff740d0b87205