Revise GSI targets to inherit from mainline_system directly

The patch seperates gsi_common.mk to aosp_product.mk and
gsi_release.mk. Let GSI targets can be inherit with 3 parts:

- system: mainline_system.mk
- product: aosp_product.mk
- vendor: none or emulator vendor makefiles

aosp_product.mk includes the AOSP packages/settings will be in
the product partition.

GSI targets also includes the gsi_release.mk, which contains
the special settings for the GSI for releasing, ex.
skip_mount.cfg, extra VNDK, etc. After the patch, the build
targets inherit from aosp_$arch.mk will not apply these special
settings anymore.

The gsi_common.mk, legacy_gsi_common.mk and aosp_$arch_ab.mk will
be phased out later.

Bug: 135502030
Test: build gsi_arm64-userdebug and aosp_arm64-userdebug, compare
Test: root, system and vendor in out folder with/without the patch
Change-Id: Ic604ac91ba9a704e6507bac324d19bbf68063464