Make Bitmap#setNinePatchChunk private

Bug: 150395371
Test: make

This is no longer used internally. Remove @hide so no one is tempted to
use it. Leave the method here out of caution - it is marked
@UnsupportedAppUsage and static analysis shows some usage (though
runtime analysis shows none). Though it's likely unnecessary to keep,
there is little to no burden to keeping it.

Change-Id: I2f2f7b6cec45630acde1e0d903ca5dc6a3fec349