Extract ClientState to a top level class

This CL represents a preparation step for FocusController.

This is a pure mechanical refactoring (moving mCurFocusedWindow to
FocusController), therefore no feature flag is required.

Bug: 314150112
Test: atest CtsInputMethodTestCases FrameworksServicesTests
Test: atest FrameworksInputMethodSystemServerTests
Test: atest --host FrameworksInputMethodSystemServerTests_host
Change-Id: Id650f03d9f20c842062018aa20913726d291ea85