Reduce max BT device length to 246 bytes due to Bluez bug. (DO NOT MERGE)

The maximum Bluetooth device name length is 248 bytes. There are bugs in
Bluez and D-Bus that can cause the Bluetooth service to go into a reboot
loop when the device name is set to the maximum length. Changed Settings
app to limit the device name length to 246 bytes.

Bug: 3246147
Change-Id: I2f46303cf44951f0b3907c43d4b096736f08765b