Re-use SpanSets in TextLine

TextLine objects are already stored in a pool to limit allocation.

Associate SpanSet objects to each TextLine to further limit the
allocation of these and re-use their internal arrays (re-sized if needed).

Drastically reduces new objects allocation during rendering.

Priority code removed in init: priorities are already handled by getSpans.

This is a duplicate of the already accepted
https://android-git.corp.google.com/g/#/c/153970/
but without the dependency on an other CL.

Change-Id: Iaa7e2f7a8ea4337c8d60c3a9a620e9e3e60caf12