DscpPolicy: remove post-T API dependency from test

The DscpPolicyTest needs to be merged into CTS13 R2, so it cannot use
any APIs that were introduced after the T cutoff.
This slows the tests down *slightly*. These test are extremely slow to
begin with (20+ seconds), so the slowdown is negligible.

Test: atest DscpPolicyTest
Change-Id: I8b3eb0e1b72f5e77540adefcf53a6a6885126d4e
1 file changed