Revert "remove emulator_vendor.mk from aosp_arm64.mk"

This reverts commit a780171d.

Reason for revert: This appears to be causing intermittent build
failures. Vendor is now small enough that sometimes it does not
get enough inodes allocated for the files that must be copied to it.

Bug: 155340761
Change-Id: I453101a65d4c8b5824467d0bb7cc8c3a03ca2740