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`.
this method seems dangerous; if it's _not_ hint text, it will have altered the text just by checking. we should probably only do this in a context where we don't care that we're mutating the text.