Remove redundant synchonized blocks from TSMS

As a preparation to fix Bug 63452807, this CL cleans up some redundant
synchonized blocks in TextServicesManagerService (TSMS).

This is purely a code clean-up that has no behavior change.

Bug: 63452807
Test: compile
Change-Id: I5f8c112d72dad46297b59da623f066fa944c1a27