Fix a lint error in getSortedInputMethodAndSubtypeList()

With this change the returned object from

  InputMethodSubtypeSwitchingController
      #getSortedInputMethodAndSubtypeList()

is guaranteed to be a mutable list.

There must be no observable behavior change.

Bug: 309837937
Test: presubmit
Change-Id: I751efade5ba68eeebfc1f215411d7cd34e667ad4