NPE in GMail / TextLine

Bug 5753061

https://android-git.corp.google.com/g/#/c/154756/1 revealed an
other bug in SpanSet. The fitered (non empty) spans were added
in their original position instead of being indexed by count.

The nullation on recycle hence left null holes in the array.

Change-Id: If5c1435cee9a2cb88a608aa8e5f4f2f23382154c