Only classify text when the toolbar is shown.

classifyText() is used to generate a menu item in the selection toolbar
to handle the selected text. In this cl we avoid calling classifyText()
when the selection is changing but instead call it when we are about to
show the toolbar.

Previously, we depended on invalidateActionModeAsync() to classify text
after a call to startActionMode(). Now that we've introduced
invalidateActionMode() we need to be able to tell
startSelectionActionMode() that we also want the text to be classified,
hence the introduction of an input parameter, "adjustSelection", to
startSelectionActionModeAysnc().

Test: bit FrameworksCoreTests:android.widget.TextViewActivityTest
Bug: 34966796
Change-Id: I5b9fc9e8ab443f024f8ca461df5a4bcc6485d26b