Switch to the previous IME switcher dialog title

This CL logically reverts a previous CL [1] that changed the dialog
title of IME switcher dialog from "Choose input methods" to "Change
keyboard".  After some discussion, UX concluded that the previous
title makes more sense because an IME is not necessarily a "keyboard".
Voice IME is a clear example.

 [1]: I8629d04eabf7eaacdcb81162fab035ba5f70a51d
      8e2a1dfa.

Fixes: 17906658
Test: manually done
Change-Id: I50f9fbf8affe7e60455105e2ff2f7e427e92cbfc