Remove the internal APIs of TSM to change spell checker settings

Previously, TextServicesManager was exposing APIs to system services to
modify spell checker settings. However, Settings application is the only
client of these APIs and there is no need to expose it to all the other
services. Settings app already has the WRITE_SECURE_SETTINGS permission
and can directly update the spell checker settings. Hence, we can remove
these APIs from TSM.

Fixes: 62950392
Test: Manually as follows.
      1. Build and flash an OS image.
      2. Complete the setup wizard (if any).
      3. Make sure AOSP Keyboard (com.android.inputmethod.latin) is installed
      4. Install SampleSpellCheckerService
       4.1 tapas SampleSpellCheckerService
       4.2. make -j
       4.3. adb install -r out/target/product/generic/system/app/SampleSpellCheckerService/SampleSpellCheckerService.apk
      5. Set the current spell checker service to be AOSP SCS by
       adb shell settings put secure selected_spell_checker com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService
      6. Run a test program that has TextView and tap on one of the
       TextViews and type some text.
      7. Observe that there is a connection to AOSP SCS by
       adb shell dumpsys textservices
      8. Set the current spell checker service to be
      SampleSpellCheckerService SCS by
adb shell settings put secure selected_spell_checker com.example.android.samplespellcheckerservice/.SampleSpellCheckerService
      9. Observe that there is a connection to SampleSpellCheckerService
      SCS by
       adb shell dumpsys textservices
Change-Id: I4513ca661788f00785f684c21d7244c233b6e33e