Remove dependency on IME subtype from TSMS

Now that we are going to reduce the dependency on InputMethodSubtype
everywhere, TextServicesManagerService#getCurrentSpellCheckerSubtype()
is a good candidate to clean up.

Given that imm.getCurrentInputMethodSubtype().getLocale() is not that
reliable, always using the fallback logic to use the system locale is
less likely to make things worse.

Also, directly calling IME API from TextServicesManagerService have
caused multiple issues like Bug 28889203 and Bug 31273203.  Removing
this API call should have positive impact on code health and system
stability.

Fix: 110275412
Test: Manually verified that spell checker still works
Change-Id: Ic222b21ce5ec4fc17b338eb5519e6928d853c8b6