Add debug log for showSoftInput

As CL[1] added the reason field in showSoftInput to expose where the
show request comes from.

Refine the original showSoftInput debug log from CL[2] to add the
reason for helping the issue clarification.

[1]: I390dc029e7bcc30c200926a9bfbbbd0268a1f714
[2]: I67a790f9d98d6131aaf04e4bb98d2a28873d3424

Bug: 182071625
Bug: 171827834
Test: adb logcat -s InputMethodManager:D
Change-Id: I73ab4e2b856669b6b6922ee451da1819e5ca18f4