TextLinks API updates.

1. Change TextLinks.getText() return type from String to CharSequence
2. Javadoc improvement

Bug: 147761091
Test: atest FrameworksCoreTests:android.view.textclassifier
Test: atest android.view.textclassifier.cts.TextClassifierValueObjectsTest
Change-Id: I40e4f401025f96717272a9ea2aac4c168297191e