Remove allowBlocking from BluetoothManagerService

Since Bluetooth is becoming a mainline module, it can no longer call the
allowBlocking hidden api.
All preliminary work as been done in aosp/1943964. We just forgot to
remove the call to the hidden method.

Bug: 211851706
Test: build + start bt
Tag: #refactor
Change-Id: I810cf7560c5cd1bf7a06ee2612b43b82e4016928