Call Sequencing: Internal cleanup and unit tests

This CL performs some cleanup on the initial call sequencing
implementation to better separate logic across classes. All sequencing
related implementation details have been moved into
CallSequencingController. CallsManagerCallSequencingAdapter serves as
the delegator between routing between the sequencing and the legacy
code path.

Unit tests have also been added in order to test the sequencing code
path.

Bug: 383395139
Flag: com.android.server.telecom.flags.enable_call_sequencing
Test: atest TelecomUnitTests
Test: atest CtsTelecomCujTestCases

Change-Id: I8d7c14d240fbc68be8226b9e2879625e9dedb19d