Handle hint text when setting text in Android

Fix issue where hint text is prepended to text being set in an EditText field. Also clean up `Clear` logic. 

`Clear` will still fail when there is hint text, since even after doing everything to clear the text, the `getText` method gets the hint text. There is, therefore, no way to distinguish a failure to clear, and the presence of the hint text. It's unfortunate that there is no way to get the underlying `EditText` object from the `UIObject`.
If this is run only after the other clear methods, then the non-hint text should already be removed.