Use StringJoiner in InputMethodDebug

This is a follow up CL to my previous CL [1], which added
InputMethodDebug#softInputModeToString().

It turns out that there is already a nice utility class
java.util.StringJoiner that can deal with delimiter when joining
multiple string tokens.

This CL also adds some simple unit tests for InputMethodDebug.

 [1]: I83f4795e95bc2e8ae325ad6e28d3a42317414e8d
      a468d70e

Bug: 118040692
Test: atest FrameworksCoreTests:com.android.internal.inputmethod.InputMethodDebugTest
Change-Id: I55fcdecee108b3e56bf7fd480d6f71de580dcbf6