Stop pooling TextUtils.SimpleStringSplitter in InputMethodSettings

This is a mechanical refactoring CL that removes the following object
pools.

  InputMethodSettings#mInputMethodSplitter
  InputMethodSettings#mSubtypeSplitter

There must be no observable behavior change.

This is a preparation to decouple InputMethodSettings from
InputMethodManagerService global instance.

Bug: 309837937
Test: presubmit
Change-Id: I915fa3a5405944a82cfa949d9821739964c8e9c9