Reduce ClientController#mClients visibility scope

Bug: 314150112
Test: atest CtsInputMethodTestCases CtsInputMethodInstallTestCases
Test: atest FrameworksImeTests FrameworksInputMethodSystemServerTests
Test: atest --host FrameworksInputMethodSystemServerTests_host
Change-Id: I3d976961846bc4733686cc121a2a3aa9a73bb75c
3 files changed