Allow 0sp text size in xml for TextView

Before this CL, when TextView's textSize attribute was set to 0sp in
XML, the text would still be visible on the screen, as the actual
textSize set was non zero. On the other hand, if the text size was set
to 0sp programmatically, the text would not be visible. This was a P
regression, as on O the text would be invisible in both cases.

This CL fixes the attribute reading stage in TextView, allowing the
application of a 0 text size on the view.

Bug: 110251171
Test: atest CtsWidgetTestCases:android.widget.cts.TextViewTest
Change-Id: I3798361e182f45a67cd0a69d40e09e559375aa20