Revise aosp_x86_arm to inherit from mainline_system directly

After applying the patch, the contents in system/product align
to the mainline_system/aosp_product.

aosp_x86_arm is not releaseded for GSI. The patch also inherits
from BoardConfigMainlineCommon.mk instead of
BoardConfigGsiCommon.mk. But keeps output system_ext and product
files to /system/system_ext/ and /system/product/.

Bug: 136616665
Test: Compare the product out folder with/without the patch
Change-Id: I2aefbd2d3e71e26219508e53161fa8a3cc2fd652