Lazy-load textclassifier settings.

Re-enables reading settings from device_config.
See: I6b7ab56e4015448ee068deb49e7f6fa133fea53c
Updates tests.
Updates documentation.

Bug: 129934185
Test: atest android.view.textclassifier
Test: (Performance) Test: frameworks/base/apct-tests/perftests/textclassifier/run.sh
      Compare performance results with ConfigParser.ENABLE_DEVICE_CONFIG
      set to true vs false. Trivial regression recorded i.e. 1.03x.
Test: (Manual) Change flags and see them reflected. e.g.
      adb shell cmd device_config put textclassifier system_textclassifier_enabled false
      Verify that app no longer uses the OEM TCS but the AOSP TC.
Change-Id: I4c6ff781c97fc2e3d3da55dc49123fa1d759670a