Fix BLE scan leaks

If an app starts multiple scans with same callback then
there is a collision in mLeScanClients hash map which
results in leaking first scan. This change fixes it by
not allowing the second scan with same callback.

BUG: 62389939
BUG: 38198694

Test: Tested applications which started multiple scans with
       same callback.

Change-Id: I569069a40b6f8b4b8bb070731225e732c6b23ec8