Fix memory corruption of BLE whitelist hashmap

When adding devices to the whitelist for LE background connections, a
local variable is used as the hashmap key, preventing any successful
lookups going forward. Thus the device will repeatedly add the same
device to the whitelist and preventing successful connections going
forward.

Bug: 23423602
Change-Id: I3d2590c0a1dd66c6e1864ea53f875a713660b645