Minimize amount of Flow Control packets for L2CAP CoC

In order to not waste bandwidth on Flow Control packets with credits:
1. Set Initial Credits in the connect request/response to the maximum value
2. Send credits only when the remote has less than 64 of them left,
   increasing the value back to maximum.

Bug: 68359837
Test: observe data flow in btsnoop log
Change-Id: I34e1fecc55422b34df0b1e82bb2b402eb06620bd