Ensure access to mDisconnectedToneBtFutures is synchronized.

The original separate bind to BT ICS code didn't sync the access to
`mDisconnectedToneBtFutures`.  Synchronizing that on `mLock` will prevent
this type of NPE.

Flag: NONE trivial npe fix
Fixes: 373711813
Test: Run CTS tests, unit tests; this isn't really reproducible however as
it is an uncommon timing issue.
Change-Id: I1ae8d2869073428bebe7653a35fc7a5e30d973f1

Change-Id: I8adb371dbf5dd3a2b5da3319e7e33639a8baf285