Move some methods from InputMethodClient to InputMethodDebug

This is the first step to split InputMethodClient into multiple
classes.

With this CL, utility methods to convert integer constants to String
messages will be moved from InputMethodClient to InputMethodDebug,
which I believe is a bit more descriptive class name than
InputMethodClient.

This is a mechanical refactoring. There should be no user-visible
behavior change.

Bug: 118040692
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest FrameworksCoreTests:android.view.inputmethod.InputMethodManagerTest
Test: prebuilts/checkstyle/checkstyle.py -f \
      frameworks/base/core/java/com/android/internal/inputmethod/InputMethodDebug.java
Change-Id: I83f4795e95bc2e8ae325ad6e28d3a42317414e8d