Track the last softInputMode in IMMS

A field in WindowManager.LayoutParams softInputMode is something that
definitely needs to be kept tracking in historical debugging
infrastructure across IME-related processes (Bug 35079353) [1].  As a
preparation, this CL enables InputMethodManagerService (IMMS) to
include the last softInputMode specified in IMMS#windowGainedFocus()
in the dumpsys in human readable format.

 [1]: As explained in b.android.com/224318, softInputMode misspecified
      by app developers is a typical root cause of unexpected behavior
      in keyboard visibility.  Bugs such as Bug 23168250, Bug 27275709,
      and Bug 31770400 fall into this category.

Test: `adb shell dumpsys input_method | grep softInputMode=`
Bug: 35079353
Change-Id: I485ced030def179dad78b4b811c6eb52b5e5c951