Speculative fix for concurrency SessionManager issue

Change the constraints for the session cleanup runnable to not
synchronize on the global instance. Instead, synchronize on the
Handler object to remove pressure on the hot path.

Flag: com.android.internal.telephony.flags.ims_resolver_user_aware
Bug: 375088362
Test: atest TelecomUnitTests
Change-Id: I65b82c1c8b74367d5611f6c3582a27073a2af6f2