optimize low-hanging fruit found using traceview

* View.getTag/setTag is a slow way of finding an overlay view;
  switch to a SparseArray
* this also obsoletes mChildrenToRemove since we can immediately
  remove overlays from the SparseArray
* bring back the Email Address cache to avoid repeated parsing
* avoid repeatedly calling findViewById for each header
* cache expensive timestamps and recipient summary strings

Change-Id: Ic76265848e4956a28a44c4a2e3e726ec9ef6bfaf
6 files changed