Fix EditText RenderNode invalidation bugs

EditText uses a TextDisplayList data structure to hold RenderNode
objects for optimized incremental drawing. This data structure uses an
isDirty flag to indicate when it has been invalidated and needs to be
re-rendered. This flag was not being computed correctly, leading to
excessive re-rendering.

This patch clears isDirty after rendering text into the RenderNode, so
that it can be reused until it is invalidated, and also explicitly sets
it when it is recycled.

Bug: 19371378
Change-Id: I70239cc30e43bb8631dfffc2ea2705e8d4c452f4