Clear UCE server state when RCS ImsService disconnects

When RCS ImsService feature disconnects, do not carry over
the UCE server state, since we are connecting to a new stack.

This is important during CTS tests, where we set server state
during testing and do not want it to carry over between tests.

Test: atest CtsTelephonyTestCases:RcsUceAdapterTest
Change-Id: I6234bb640e2cd1083d327f7ce46c77b011fb0cee