Adjust method access levels in InputMethodUtils

This is a mechanical refactoring that adjusts access levels for
methods in InputMethodUtils.

With a recent CL [1], which moved InputMethodUtils to services.jar,
some of methods in InputMethodUtils can now have strict access level
than before.

 [1]: I37594096151d77adb6741a41d7985a4506f96ccb
      e6b6e0e7

Bug: 123379418
Test: make -j checkbuild
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Iaf737787426d3a67df8ad6595c2b094fad139619