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 VtsHalTvTunerV1_1TargetTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e7c5595ee5cfe79eb181e60399d8d857526d1af7)
Merged-In: I62ad038829ca3ed0ecc077527197eec444a5eb12
Change-Id: I62ad038829ca3ed0ecc077527197eec444a5eb12