Move buildInputMethodsAndSubtypesString to InputMethodUtils

This CL is mechanical code moving and does not change any existing
behavior.

buildInputMethodsAndSubtypesString is introduced by
If0104151b3526da6ecc669adde3119a239ecafeb for addressing Bug 19822542.
This code moving is one of the TODOs in above change.

Bug: 22285167
Change-Id: Ie63cf593794c9062919887e04a64208a900b1b8b