[Thread] start / initialize Thread service early

We need to do initialization work and start OT daemon when
system_server boots. But the initialize() is called at the phase
PHASE_BOOT_COMPLETED which is too late as Apps (e.g. Google Play
Services) can be started before this phase.

This commit changes to start the Thread service earlier at the phase
PHASE_SYSTEM_SERVICES_READY. Other services such as Wi-Fi, Bluetooth,
Ethernet and Nearby all starts / initializes at this phase.

This commit resolves comment aosp/2737373/comment/d65b9446_8a08117c

Bug: 318787467
Test: atest CtsThreadNetworkTestCases
Change-Id: I7b7b8551b9114cbf07f995b64316c7b420d868b6