Move the drawable state to the correct class.

Bug #4170455

The state tracking whether the Paint's shader should be rebuilt was kept in the
wrong class. This lead to the Drawable keeping track of the dirty status of the
Paint stored in the ConstantState. This of course does not work properly when
several drawables are inflated form the same ConstantState. This change fixes
the issue by moving the dirty Paint state to the ConstantState class. This is
allowed to work because of the single-thread rule enforced by the UI toolkit.

Change-Id: I9bb31d8e7335d6bb418470b59ae25d9085d7bd23