Add a mutex to guard all btsnoop operations

After switching to HIDL, capture() can be called by multiple threads.
Capturing a packet and writing to the btsnoop/btsnooz logs
require multiple operations, hence we need a mutex to make those
operations atomic.
Previously, there were occasional btsnoop_hci.log files
with corrupted data as reported by Wireshark.

Test: Manual - captured multiple btsnoops_hci.log files.
Change-Id: Ib89d9156ad7e27a7dee652f71a0cf38c85d4447c