Define PRODUCT_SYSTEM_* props for mainline_system

These properties, namely:

- PRODUCT_SYSTEM_NAME
- PRODUCT_SYSTEM_BRAND
- PRODUCT_SYSTEM_MANUFACTURER
- PRODUCT_SYSTEM_MODEL
- PRODUCT_SYSTEM_DEVICE

propagate into /system/build.prop as ro.product.system.* and can
identify if the /system partition is mainline_system.
These don't modify other ro.product.* properties that are defined in
partitions other than /system.

Bug: 110206836
Bug: 146191296
Test: lunch mainline_system_arm64-userdebug; m
Test: lunch aosp_arm64-userdebug; m
Test: Two products must have same PRODUCT_SYSTEM_* props
Change-Id: I300579d555d269481635237fb02aab5db71a510d