Clear native shader when Paint shader changes

Since Recents never goes away that means TaskViewThumbnail
never goes away which in turn means mDrawPaint never goes away.

mDrawPaint has a setShader(null) called on it to release
the BitmapShader it was previously drawn with, but that
change didn't push down to native until the next time the
paint was used. As a result, the Paint held on to a
reference to the bitmap.

The fix is simple, when the shader is changed simply
push down an immediate clear of the native shader. Since
the bitmap of a BitmapShader isn't changeable we don't
need to worry about clearing references for shader mutation,
merely when Paint#setShader() happens.

Bug: 38258699
Test: Open & close recents, observe EGL memory immediately returns
to sub-5MB.

Change-Id: I6742efa8ae8dfca76b62a06bc1a1a00646feba64