Move FakeDns out of NetworkMonitorTest.

This ultimately belongs in p/m/C/staticlibs/testutils, but there's
already a FakeDns class there. Move it out first, and then unify
the two in a later CL.

This is a no-op code move.

Test: test-only change
Bug: 240259333
Change-Id: I61a4877f21bd4c7b9d5ba2842bf122878be66344
2 files changed