Extract StartInputReason from InputMethodClient

This is another step to split InputMethodClient into multiple classes.

With this CL, StartInputReason will be extracted from
InputMethodClient.java into a dedicated file.

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/StartInputReason.java
Change-Id: I0cc2588c97239a004720f74cbf356bda4c735d53