sensors: fix concurrent modification exception

The sensor listener can be removed in unregisterListener
inside of the for loop.  This change makes a copy of the
listeners and iterates over the copy.

Test: builds
Bug: 37543280
Change-Id: Ie9b7b9afb6994d9627f67c4b3bfbdfb305f5583e
Signed-off-by: Daniel Moore <dtmoore916@gmail.com>