GpuMem: fix flaky unittests on cuttlefish platform

There isn't enough ram allocated in the default test config, which is
fixed here. This change also adds the SKIP_IF_BPF_NOT_SUPPORTED micro as
so far there's no guarantee that S will require a BPF capable kernel.

Bug: 158873003
Test: atest gpuservice_unittest:GpuMemTest
Change-Id: Id8127a4aba894da82a8c7fa3051e6748f0b61821
Merged-In: Id8127a4aba894da82a8c7fa3051e6748f0b61821