Return actual payload (DHCPDISCOVER) bytes array when starting FILS.

Generally "ByteBuffer packet" has been allocated with a maximum
length of 1500. Calling packet.array() is to return a byte array
of length 1500 but the actual DHCPDISCOVER packet payload is much
less. Return actual DHCP packet byte array instead in this patch,
which allows caller use the payload byte array directly without
extra operation (e.g. calling packet.remaining() to get the correct
length of actual payload).

Bug: 140223017
Test: atest NetworkStackTests NetworkStackIntegrationTests
Change-Id: Ib08f2450fb649ea1ffcdbf8dedc829965f13da86