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

Merged-In: Ib9b786c7d89f3b50f0338f8d10539c65101c1c64
Merged-In: I1fbca5934519fca9bd0fd993e87bdf1730524b68
Change-Id: Id1b9246bfa192683abbde466d28639cbe6f94483
(cherry picked from commit 0bcb3748)