Clarify Drawable.getOpacity() docs

Docs for translucent-vs-transparent-vs-opaque were confusing, especially since
the definition for those constants in PixelFormat are not the same as how they're
actually used in Drawable. This fix simply adds clarifying text to the method
comment for getOpacity().

Issue #21158891  Better document Drawable#getOpacity

Change-Id: I94725592f85e5d7874e3a3ac1c5bab969163fbf0