Fix minor WinRT backend bugs

[devanlai]

This pull request addresses some minor issues I found while trying out the WinRT backend, as documented in #536. After some gentle ribbing from @dlech, I decided to roll these changes up into a pull request.

For me, these changes were sufficient to allow me to scan for a custom BLE device using the local name as a scan filter, connect to it, and interact with the characteristics.

I ended up making three changes:

1. Updating the WinRT backend to implement the discovered_devices property instead of the old get_discovered_devices() method.
2. Fixing an assertion in the UUID construction when the device has 128-bit service UUIDs.
3. Fixing a typo in the advertisement filter attribute of watcher objects

For reference, I was able to filter on the local name with a variant on the following code:

from winrt.windows.devices.bluetooth.advertisement import BluetoothLEAdvertisementFilter, BluetoothLEAdvertisementBytePattern
from winrt.windows.security.cryptography import CryptographicBuffer

# Match on the complete local name field (0x09)
local_name = "mydevice"
adv_filter = BluetoothLEAdvertisementFilter()
local_name_bytes = CryptographicBuffer.create_from_byte_array(list(bytearray(local_name.encode("ascii"))))
name_byte_pattern = BluetoothLEAdvertisementBytePattern(0x09, 0, local_name_bytes)
adv_filter.byte_patterns.append(name_byte_pattern)

devices = await bleak.BleakScanner.discover(AdvertisementFilter=adv_filter)

If you'd prefer, I can squash these changes into a single commit.

[dlech]

Looks good to me. Thanks for the fixes!