Add ClipDescription method to determine if the clip is styled text.

This adds a new method to ClipDescription to allow callers to
determine if the associated ClipData is styled text, without
accessing the ClipData directly.

This information is used by editors such as TextView or WebView
when they show a selection toolbar, in order to determine if a
"Paste as plain text" option should be shown. Currently, ClipData
must be accessed in order to do this, so a clipboard access
occurs whenever text is selected.

Avoiding these clipboard accesses is desirable because a user-visible
message may in future be shown whenever ClipData is accessed
(see b/167676460), which may cause user confusion.

Bug: 167660455
Test: new tests added to ClipDescriptionTest in CTS
Change-Id: I48bea4d11a68b198d60016b364a3cfa0b0bf277e