Separate LE scanner from GATT client (2/4)

Right now, LE scanning functionality is combined with the GATT client.
This is the source of various bugs, like scans suddenly stoppinging when
a GATT client is killed. It also increases memory consumption, because
we associate many structures with a GATT client, which are not necessary
when just scanning.

Test: sl4a BleScanApiTest ConcurrentBleScanTest
Bug: 30622771
Change-Id: Ie4a20abe7fa112bf501619651e288ae46d28e209