Remove redundant prefix from StartInputReason

This is another step to split InputMethodClient into multiple classes.

Now that all the integer constants for StartInputReason are defined
inside StartInputReason, "START_INPUT_REASON_" prefix is just
redundant.

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/StartInputReason.java
Change-Id: Ic2476c3d588211e6c61180cde7df4c6b79039ede