Fix Bluetooth set-test-mode flakiness

The current set-test-mode* handler implementation intermittently stays
active (i.e., behaves asynchronously, and sometimes leaks) after being
disabled. Fix it by making the code more robust and addressing thread-
safety issues.

* Some background: we recently we added the ability to temporarily
place the Bluetooth service into a special state, called
"set-test-mode", via the command line. When in this state, all
Bluetooth scans will return "fake", hardcoded, Bluetooth scan results.

Bug: 186437226
Bug: 188070143
Test: atest --rerun-until-failure=1000 NearbyDevicesPermissionTest PermissionTest30WithBluetooth
Change-Id: I527fe7b0539c3c191c4318e5dbb21284b491f35e