Let LinkProperties#getRoutes() keeps returning all routes before T

Starting from T, VpnService supports exclude routes, which will
make the caller to get both of include routes and exclude routes
from LinkProperties#getRoutes(), and it's not expected to get the
exclude routes before T in production code even though the code
returns all routes.

But there is a CTS - LinkPropertiesTest#testRouteAddWithSameKey()
will try to add an exclude route and check if the result of
LinkProperties#getRoutes() contains that exclude route or not.
And the test is failed now since LinkProperties#getRoutes() will
only return include route if compat feature - EXCLUDED_ROUTES is
disabled. (EXCLUDED_ROUTES is enabled starting from target SDK T)

To fix this test failure, let LinkProperties#getRoutes() keeps
returning all routes if SDK is before T. This avoids changing
behavior on S, and even though there is still a behavior change
when upgrading to T, apps are unlikely to be relying on such
behavior, especially outside of tests.

Bug: 238061814
Test: Run "atest CtsNetTestCases:LinkPropertiesTest" on S build
      with mainline module which contains this patch.
Change-Id: Iac4362c4fe347ee3f06d5b21b0325fa69a7f27b6