Fix a JavaDoc link to inputMethodClientHasFocus()

I forgot to update this JavaDoc in my recent CL [1] that moved
inputMethodClientHasFocus() from WindowManagerService to
WindowManagerInternal.

 [1]: I20c618174d8279a9a57f458fb908235f452b2281
      cf93f9a5

Bug: 112722651
Test: compile
Change-Id: I46cc9dd7479cc178770098a7bb6f0785598db3cf