L2cap ERTM: Enter local busy on EnqueueBuffer full

* Allow EnqueueBuffer user to check current buffer size and register for
a callback when buffer is empty.
* In ERTM DataController, enter/clear local busy condition when it
detects EnqueueBuffer full (contains 2 items so far). Note that there is
a buffer in Queue itself.
* Add corresponding PTS test case.

Test: cert/run --host
Change-Id: Id2f7e745f010cbebfdc2ed61920ad3baec7968f3