Add new products aosp_x86_a and aosp_x86_64_a

The product is for GSI with x86 64-bits and 32-bits without A/B
update.

Bug: 38339837
Test: Build pass and boot to home screen on 32/64-bits x86 emulator
      by replacing system image with GSI
Change-Id: I9f956bc35ba5700900df7c2fca45bf794684a418