ServiceStateTracker: remove synchronization from mSignalRequestRecords

With all access pathes of mSignalRequestRecords in the same thread now,
the synchronization with lock hold on itself can be removed. This
will bring better readability and performance.

Bug: 179077640
Test: atest com.android.internal.telephony.ServiceStateTrackerTest
Test: Manual feature test
Change-Id: Ice391f48c16e3233fb1628093e469345a900bf5a