speech: Remove hard-coded preference key from the controllers

OnDeviceRecognitionPreferenceController and TtsPreferenceController
had their preference keys hard-coded which was breaking
AccessibilitySettingsTest. The key is now left to be read
from the super constructor.

Bug: 283449837
Test: Manual, already existing robotests
Change-Id: Ibe11a2134fffea0a081ca9f5e2f9441911574efa