[bt][l2cap] ERTM TxEngine: honor MaxTransmit for RR-Poll

Use the MaxTransmit parameter to limit the number of times we
will transmit a receiver-ready poll.

* Per Section 5.4, when MaxTransmit=1, only one transmission is
  permitted.
* Per Section 5.4, when MaxTransmit=0, the poll should be
  retransmitted indefinitely.
* Per Section 8.6.5.8 (see second to last row in table), when the
  monitor timer expires, and we have _not_ exhausted MaxTransmit, we
  should retransmit the poll frame, and restart the monitor timer.
* Per Section 5.4, as well as Section 8.6.5.8 (see last row in table),
  when the monitor timer expires, and we have exhausted MaxTransmit,
  the connection should be closed.

(References above are to Core Spec v5.0, Volume 3, Part A.)

Bug: BT-442
Test: bt-host-unittests

Change-Id: I15e73ab50a8cc8ccaf1f678db5f709350637bf50