BT: Limiting the thread lock to critical part of state update

Issue: ANR occurs due to UI wait for long time waiting to
acquire thread lock.

Cause: The synchronized function that updates state also
read paired devices as an additional operation. When the
number of devices is cached list is large, the block time
for other threads waiting for same lock tends to increase
causing ANR in case of UI thread.

Measure: Limited the synchronized block to the part where
actual update of local state takes place.

CRs-Fixed: 1053291
Change-Id: I69ff9f8a032b3772bf3d048d8db70181319ad31d