Add Paint#get(ShadowLayer)ColorLong

Bug: 120904891
Test: I6de01bd94fade605643af590f8d6909f09a5831e

Store Paint's colors (both the ShadowLayerColor, and the previously
uncached color that is stored in the native SkPaint) as @ColorLongs.
When setting or retrieving the int version, convert. This allows
returning the long value that was manually set, so that we return the
color in the ColorSpace that was requested.

Since the color is already stored in Java, remove nGetAlpha, and return
the alpha from the ColorLong. When setting alpha, update the cached
value, too.

Make setShadowLayer(..., @ColorInt) and setColor(@ColorInt) call the
@ColorLong versions, so they can share single JNI entry points.

Change-Id: Ifc559893dd4db2629c59b6e53f0b2166d43e6049