Cache Integer.toString results for small numbers

This avoids repetitively creating String object for numbers less than
100, significantly improves performance while has small impact on
memory usage.

    === Angler a57@1.4GHz ===
               benchmark  Orig  Mod
_IntegerToString10_small  1636 29.8
  _IntegerToString_small  1607 18.8

Bug: 25887112
Change-Id: Ibdd6556c8f2a99f0dce9b0194934eeb23931553d
1 file changed