Update TextServicesManager API.

- Rename getCurrentSpellChecker to getCurrentSpellCheckerInfo.
- Rename getEnabledSpellCheckersList to getEnabledSpellCheckerInfos.
- Remove getCurrentSpellCheckerSubtype.

This CL is effectively rewriting I2bac96ddafe5e4ac583fc16bf155d67e3d45fc4f
to introduce the following two methods as official APIs.
- getCurrentSpellCheckerInfo()
- getEnabledSpellCheckerInfos()
This CL adds back @UnsupportedAppUsage that were removed in the commit
above.

Bug: 179804789
Test: atest CtsInputMethodTestCases:SpellCheckerTest
Change-Id: Ibf5fbf5fe6f58f999c4af40ae937d68e71def766