Add a flag to allow system health experiment for the behavior change ...

of getDefaultTextClassifierImplementation()

Once the system health experiment is done, revert this change and
remove the local TC.

Test: Manual
1. Set use_default_system_text_classifier_as_default_impl to be false
   by using device_config command. Select a phone number.
   Verify the phone action is shown. Verify dumpsys textclassification
   shows that default TC is bound.

2. Set use_default_system_text_classifier_as_default_impl to be true.
   Reboot the device.
   Select a phone number and verify the phone action is shown.
   Verify dumpsys textclassification shows that the default TC
   is not bound.

BUG: 148049185
Change-Id: I602c3cb6d7bef468cba50be6ed8d799d79af8240