Introduce InputMethodUtils#concatEnabledImeIds()

This CL introduces a stateless utility method

  InputMethodUtils#concatEnabledImeIds(),

which can be used as a replacement of

  InputMethodSettings#appendAndPutEnabledInputMethodLocked()

so that we can later remove

  InputMethodSettings#mEnabledInputMethodsStrCache

to make InputMethodSettings a thin wrapper of mMethodMap.

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

Bug: 309837937
Test: atest InputMethodUtilsTest#testConcatEnabledImeIds
Change-Id: I01d13d1f46a5af19d6db18246f74a20a4fe14168