Generic AOSP system image for non-A/B arm devices

aosp_arm_a: for non-A/B 32-bit arm devices
aosp_arm64_a: for non-A/B 64-bit arm devices
Both are based on aosp_arm64_ab (for A/B devices).

Bug: 36153731
Test: build with oc-dev and the resulted system image works on one
32-bit arm device successfully (no 64-bit arm device to test yet).
Change-Id: Ifb5c4239e5c22005e055cfa840feca9c0a92586f