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: I97e215d3412d4b61757d6dc011b90cc5f2c8f084