Fix deadlock in CachedBluetoothDeviceManager

Following deadlock exist in previous code:

1. Main thread:
Device.refresh() -> Device.dispatchCallback() -> Lock callback
-> (handle callback in systemui) -> DeviceManager.getDevicesCopy()
-> try lock deviceManager

2. Bg thread:
DeviceManager.onActiveDeviceChanged() -> Lock deviceManager ->
Device.dispatchCallback() -> try lock callback

This CL remove sychnorized method in DeviceManager to unblock this
deadlock.

Fixes: 143373496
Test: Manual
Change-Id: Ib17d99a8d797e4e514e965541a04a520f89edad5