Fix frame-allocated path lifecycles

bug:18667472

Previously, we were allocating per-frame temporary paths within the
PlaybackStateStruct, but these are not safe as layers allocate these
transiently. Instead, move these to the OpenGLRenderer, which has
better define lifecycle.

Additionally, don't store SkPath objects directly in vector, since
they are then subject to relocation.

Change-Id: I8187ef542fcd5b030502bb75eb123ee26c0daa96