Introduce a default Voice IME concept.

This is configured by the system config resource,
config_systemSpeechRecognizer, which also provides the default
VoiceRecognitionService and the holder for the SYSTEM_SPEECH_RECOGNIZER
role.

InputMethodManagerService updates the new DEFAULT_VOICE_INPUT_METHOD
setting and handles changes to the config_systemSpeechRecognizer value.

No updates are made through the Settings UpgradeController because any
updates that would be needed are already handled by the logic for config
value changes.

Testing:
1. Enable DEBUG logging in InputMethodManagerService.
2. $ m -j && adb remount && adb shell stop && adb sync && adb shell start
3. $ adb shell settings get secure enabled_input_methods; \
 adb shell settings get secure default_input_method; \
 adb shell settings get secure disabled_system_input_methods; \
 adb shell settings get secure default_voice_input_method
4. Check logcat to make sure nothing looks suspect.

Cases tested:
- IME wasn't already in the enabled IME list
- IME was already enabled
- no value for config_systemSpeechRecognizer
- new user added
- locale changed
- config package doesn't have an IME
- update without config value, then set a config value
- config value updated (when both values had valid IMEs)
- combinations of the above cases, as appropriate

Bug: 175480456
Test: manual - see above
Test: atest InputMethodUtilsTest
Test: atest CtsInputMethodTestCases --retry-any-failure
Change-Id: I1abdc145e3d5969fbb69811df2ca2e35c7a177e1