ANDROID: Make vsock virtio packet buff size configurable

When the android virtual device stays up for long periods of time, the
SLAB sustains everincreasing fragmentation. Contributing to this
fragmentation are the virtio packet buffer allocations which are a
drain on 64Kb compound pages. Eventually these can't be allocated due to
fragmentation.

To enable successful allocations for this packet buffer, the packet
buffer's size needs to be reduced.

To enable a reduction without impacting other partners, this var is
being exposed as a command line param.

Bug: 149494272
Test: atest ManagedProfileLifecycleStressTest#testCreateStartDelete on
local cuttlefish device
Signed-off-by: Ram Muthiah <rammuthiah@google.com>
Change-Id: Ia4f9180025b64d61d9e2b882daa84460b8f08fd2