Introduce IMMS ClientController component

ClientController is the component responsible for storing and managing
IMMS clients.

This is a pure refactoring, no feature flag is required.

Bug: 314150112
Bug: 315227580
Test: atest CtsInputMethodTestCases CtsInputMethodInstallTestCases
Test: atest FrameworksInputMethodSystemServerTests
Test: atest --host FrameworksInputMethodSystemServerTests_host
Change-Id: I117396b145f098f7b852328a2f1dd3df7a9f3b28
Signed-off-by: Antonio Kantek <kanant@google.com>