DO NOT MERGE Add acceptPair method to BT stress.

Add a method to the test utils which waits to accept a bonding request from
another device.  This is identical to the pairing method except that it
waits for a remote pair request instead of initiating the pairing
request itself.  Also, add a test method to the test utils.

Change-Id: I6391d03e9f33c040afd64d73083acfe3184251eb