Move getLastInputMethodSubtype() logic to InputMethodSettings

This is a mechanical refactoring CL that moves the main logic of

 InputMethodManagerService#getLastInputMethodSubtype()

to

 InputMethodUtils.InputMethodSettings#getLastInputMethodSubtypeLocked()

without changing any developer observable behavior.

This is also a preparation to support

 InputMethodManager#getLastInputMethodSubtype()

for background users.

Bug: 234882948
Test: atest CtsInputMethodTestCases:InputMethodSubtypeTest
Change-Id: I39077b2ef170287af851e3d0e0bd51c6e668ca17