Allow L2CAP first packet with partial header

This change will allow a fragmented and first L2CAP packet that has
partial L2CAP Header to be parsed. Only the first 2 bytes of L2CAP
header are necessary for the packet assembler to work and the subsequent
bytes of the header can be in later packets.

Test: Run RfComm and LE CoC SL4A Tests. Also verify on an OTA received packet
with the partial L2CAP CoC header.
Bug: 70683224

Change-Id: I659ebbe86eab57aa9d725f7b685525b445195e5e