Add demux to mFrontendTests

aosp/2640754 adds a tuneFrontend call with testWithDemux enabled.
tuneFrontend() has some logic that calls DvrTests->openDvrInDemux()
which requires mDemux to be available, but mFrontendTests does not
have an associated demux. This CL adds a demux resource.

Bug: 317268814
Test: atest VtsHalTvTunerTargetTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e75c56787a1a9cd6280ab758788bc810ff50cb58)
Merged-In: I3eb69d8dc10cd2cdbb2d3526317faee2909787f5
Change-Id: I3eb69d8dc10cd2cdbb2d3526317faee2909787f5