Change TextView.TextAppearanceAttributes.mStyleIndex to mTextStyle

Currently mStyleIndex is used as a flag int, but its default value is -1.
Rename it to mTextStyle and change the default value to 0 in order to avoid confusion.
Also fix the bug that when fontWeight is specified while not textStyle is
specified in XML, the typeface unexpectedly becomes italic.

Bug: 78873447
Test: atest TextViewTest
Change-Id: I819b8f15e1dcaae7b16f7c97b489b4ed8db494c6