Use RemoteCallbackList to implement TSMS#mListeners

Previously, TSMS extends IBinder.DeathRecipient to implement its own
death recipient class to keep track of disconnected service listeners.
Using RemoteCallbackList simplifies the implementation.

Fixes: 35102403
Test: Manually tested 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. adb shell settings put secure selected_spell_checker com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService
      5. Observe that there are no bindings to spell
       checker service (SCS) by running
       adb shell dumpsys textservices
      6. Observe that there is no connection to SCS by running
       adb shell dumpsys activity services com.android.inputmethod.latin
       There should be no ConnectionRecord for
       AndroidSpellCheckerService
      7. Run a test program that has TextView and tap on one of the
       TextViews and type some text.
      8. Repeat steps 5 and 6 to observe there are now bindings and
       connection to SCS.
      9. Kill the app manually. Repeat steps 5 and 6 to see bindings
       and connections are removed again.
      10. Repeat 7 and 8.
      11. Kill SCS
        adb shell ps -A | grep com.android.inputmethod.latin | awk '{print $2}' | xargs adb shell kill -KILL
      12. Repeat steps 5 and 6 to observe that the bindings and the
        connection to SCS are preserved.
Change-Id: I38942765ed6bec6713757b1d5f325e7a633c2ba7