Avoid null-deref crashes in ScrollBarDrawable

Some of the code in ScrollBarDrawable automatically derefs
the horizontal and/or vertical thumb objects. The drawable can
sometimes be in a state where these objects are null, causing
a null deref. This fix simply avoids dereferencing null objects.

Also, fixed doc bug in attrs.xml for Transition tags.

Issue #13210554 Transition style documentation refers to 'move' instead of 'changeBounds'
Issue #15636059 ScrollBarDrawable crashes in setAlpha() and other methods when thumbs are null

Change-Id: I6664ba17dae0845de8a8658381ae6e9c9ef99214
2 files changed