Make min_sdk overridable in override_apex

It makes sense to make min_sdk overridable because the override_apex is
a variant of the original apex, and this variant might be
created/introduced in the newer Android release compared with the base
apex. Despite the fact that the source of the variant apex is the same
as the original one, the variant may not support the older version, like
Go modules, trimmed modules. If we consider the variant as a "new"
module, then it makes sense to have a new min_sdk to make it only
available for the newer Android released version only.

Bug: 266945903
Test: Build locally, and check if the override works
Change-Id: I7f4c7169e15b7e57f44a4abafe012151dbc226a0