Make iOS Plugin work with web-bluetooth nicely

1. Using string value of NSUUID to compare duplicate devices (Found same device
has different NSUUID).

2. Don't receive duplicate advertisement data since the delegate method runs on
MainThread. Only check removed device every 15 seconds by turn off and turn on
the scanner.

3. Do not remove device after disconnect. (web bluetooth polyfill uses connect
and disconnect to test wether a device is connectable).