Move PRODUCT_BRAND to module_common.mk

So that we don't need to specify it in individual mainline modules
products, which becomes more important after the internal ones only
inherit from the common aosp product as opposed to the arch-specific
ones.

Bug: 249685973
Test: Diff'd the soong.variables files for the module_* products and saw no differences
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:211c5c2810e50a1c5d3bca2f1f7942b624fe3abe)
Merged-In: If1733eb193cd3748fb3562d09bfd41651e2480f9
Change-Id: If1733eb193cd3748fb3562d09bfd41651e2480f9