Allow users to change the length of BT disoverability. DO NOT MERGE

It can set to be two minues, five minutes, an hour, or forever discoverable.
The default is 120 secs.
This addresses the internal bug 2413429:
	http://b/issue?id=2413429
and external issue 6348:
	http://code.google.com/p/android/issues/detail?id=6348

Original Author: Chris Wren <crwen@google.com>
Modifications done by: Jaikumar Ganesh<jaikumar@google.com>

Change-Id: Ie12e56ac41aa01a161d263c7525b6e021d4eeb1f