Add text classification results to clip description and clip data.

With this change, text classification is performed on text in clip
data when it is set as the primary clip, if it is below a certain
length.

This allows information about which entities (e.g. URLs, addresses)
are present in the text to be added to the ClipDescription.

With toast notifications being added when an app accesses clipboard,
this change will allow apps to tell if clipboard contains information
they are interested in before they access the clip data. This will
allow unnecessary clipboard accesses to be avoided.

If entities are detected in the clip data, the results of classification
are added to the ClipData so that users of this API do not then need
to repeat text classification themselves in order to locate the
entities.

Bug: 180577441
Test: new CTS tests added to ClipDescriptionTest
Change-Id: I334db4805b8b72e0e00c721fe6efe64ec0661a8a