Add support for reading a snapshot of the APF data

No functional change yet, since startReadPacketFilter() has no callers
at this time.  In the future, this new hook will be used to take
periodic snapshots of the APF memory (for instance, when the device
wakes up).

Design note: WifiStateMachine grabs the APF data synchronously
from another thread, but then the data snapshot is delivered to IpClient
via an asynchronous reply, following the same pattern used by other
commands. This means that there's no (practical) way for IpClient to
read the APF data just before replacing the APF program.

Even with this limitation, it's still possible to reliably decode packet
counters and compute deltas relative to the last snapshot, provided that
the address range isn't cleared when installing a new APF filter.

Bug: 73804303
Test: Manual - called the new code and inspected 'dumpsys wifi' output.
Change-Id: Ia0923d71cf3ee4128fb1c381557316300adac1a3
Merged-In: Ia0923d71cf3ee4128fb1c381557316300adac1a3
Merged-In: I3b940f5a3b795f85d244882eaa7eca56bd9e167d
Merged-In: I283fd5fb71f8a679911e58c487a4ac12a5190049
(cherry picked from commit bb2193bf)