DO NOT MERGE: DscpPolicy: remove post-T dependencies from test

The DscpPolicyTest needs to be merged into CTS13 R2, so it cannot
always use APIs that were introduced after the T cutoff.

Run the test when the new APIs are available, but skip it otherwise as
the feature is known to be broken unless the device is using a newer
connectivity module.

DO NOT MERGE: should not be used for U+ tests.

Test: atest DscpPolicyTest
Bug: 243891394
Bug: 243120975
Change-Id: I86eeb7a008c6e299fdf7cb709844bc336f09bdfa