Bluetooth: Protect callbacks during shutdown

- Hold the mutex during callbacks
- Disable power mode before deleting hci_

Bug: 36084266
Test: Switch users repeatedly (toggles Bluetooth)
Change-Id: I7e4d405e9028c3363c112a8b09c03b8aea652b7a
(cherry picked from commit 9ef1f7198234078095abe14c13e654504f8f2cca)
2 files changed