Switch from leaking Slice memory to MemBlock

Test: as follows
    - built
    - flashed
    - booted
    - bpfloader builds
    - system/net/tests/runtests.sh passes
    - adb shell dumpsys netd trafficcontroller shows happy BPF
Bug: 78250686

Change-Id: Id1b9246bfa192683abbde466d28639cbe6f94483