Use ArrayMap instead of HashMap for IMMS#mClients

With this CL, InputMethodManagerService#mClients is implemented with
ArrayMap instead of HashMap, which should give us a bit better memory
consumption including value enumeration without iterator objects.

One possible observable behavior change in this CL would be the order
of IME clients shown in 'adb shell dumpsys input_method'.  This should
be OK because that output is used only for debugging purpose.

Bug: 119839847
Test: No error in 'adb shell dumpsys input_method'
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Iadbe64d92d39b28338e23c5e16be44b7597deee0