Add build variant specific release configurations

They can be used to set flag values based on selected build variant.
Since this patch doesn't introduce any such flags, it's not indented to
bring any functional changes.

BUG: b/378961230
Test: Create a temporary release flag, assign different value for each
build variant and check the contents of build_flags_vendor.json.

Change-Id: Ia61efbfae8197eb378c7225ce0353b7f186665be
6 files changed