Rewrite aosp_bramble.mk to inherit aosp_product.mk

Let aosp_bramble.mk follow the CSI mainline convention.
AOSP bramble makefile now inherit mainline_system.mk
for system image and aosp_product.mk for product image.

Bug: 136525499
Test: Compare contents of $ANDROID_PRODUCT_OUT
Change-Id: I6c2bc6d2c6936bcd119bb16da24fa75e061e6086