Extract UnbindReason from InputMethodClient

This is another step to split InputMethodClient into multiple classes.

With this CL, InputMethodClient is completely removed.

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

Bug: 118040692
Test: prebuilts/checkstyle/checkstyle.py -f \
      frameworks/base/core/java/com/android/internal/inputmethod/UnbindReason.java
Change-Id: I3b96a351413025338776f6c87cbaa8cf28c3a44f