Remove InputMethodSettings#appendAndPutEnabledInputMethodLocked()

This is a mechanical cleanup CL to remove

  InputMethodSettings#appendAndPutEnabledInputMethodLocked()

by using

  InputMethodUtils#concatEnabledImeIds(),

added in my prvious CL [1].

This is a mechanical cleanup CL with a new unit test. There must be no
observable behavior change.

 [1]: I01d13d1f46a5af19d6db18246f74a20a4fe14168

Bug: 309837937
Test: presubmit
Change-Id: I6ec7b52dceb5d1334389866a565d593381119060