Make TCS.getDefaultTextClassifierImplementation returns a ...

SystemTextClassifier that is backed by ExtServices

1. The default textclassifier is always provided by
   config_servicesExtensionPackage (i.e. ExtServices)
2. OEM can specify a system text classifier by specifying
   config_defaultTextClassifierPackage.
3. System text classifiers can get an instance of the default textclassifier
   by calling TCS.getDefaultTextClassifierImplementation(), so that
   they can add their customization on top of the default TCS.
4. If config_systemTextClassifierPackage is set, the specified package
    is used to process requests from apps and the platform. Otherwise,
   the default textclassifier is used.
5. For testing and droidfooding purpose, text classifier service package
   can be overridden. If the overridden package is neither
   the default one nor the system one, the package is considered as a
   untrusted text classifier, which can only see requests from itself.

Test: m mts && mts-tradefed run mts-extservices
Test: Not setting config_defaultTextClassifierPackage. Select some text and
      make sure smart selection works. Run dumpsys textclassification
      to make sure the default TCS is bound.
Test: Setting config_defaultTextClassifierPackage and repeat the above.
      Make sure the specified OEM text classifier is used.
Test: Set the service override config to be ExtServices.
      Run dumpsys textclassification to make sure everything is unbound.
      Select some text and make sure ExtServices is bound.
Test: Set the service override to be AiAi and repeat the above.
Test: Set the service to be something invalid and repeat the above.
      Observe that no TC is bound and fallback to NO_OP.

BUG: 148049185

Change-Id: Ia2fb549fda49363e0d0ebc4b7e0d31cb76e11ee0
Make TCS.getDefaultTextClassifierImplementation returns a ...
SystemTextClassifier that is backed by ExtServices

1. The default textclassifier is always provided by
   config_servicesExtensionPackage (i.e. ExtServices)
2. OEM can specify a system text classifier by specifying
   config_defaultTextClassifierPackage.
3. System text classifiers can get an instance of the default textclassifier
   by calling TCS.getDefaultTextClassifierImplementation(), so that
   they can add their customization on top of the default TCS.
4. If config_systemTextClassifierPackage is set, the specified package
    is used to process requests from apps and the platform. Otherwise,
   the default textclassifier is used.
5. For testing and droidfooding purpose, text classifier service package
   can be overridden. If the overridden package is neither
   the default one nor the system one, the package is considered as a
   untrusted text classifier, which can only see requests from itself.

Test: m mts && mts-tradefed run mts-extservices
Test: Not setting config_defaultTextClassifierPackage. Select some text and
      make sure smart selection works. Run dumpsys textclassification
      to make sure the default TCS is bound.
Test: Setting config_defaultTextClassifierPackage and repeat the above.
      Make sure the specified OEM text classifier is used.
Test: Set the service override config to be ExtServices.
      Run dumpsys textclassification to make sure everything is unbound.
      Select some text and make sure ExtServices is bound.
Test: Set the service override to be AiAi and repeat the above.
Test: Set the service to be something invalid and repeat the above.
      Observe that no TC is bound and fallback to NO_OP.

BUG: 148049185

Change-Id: Ia2fb549fda49363e0d0ebc4b7e0d31cb76e11ee0