Consistently use Formatter's cached NumberFormat.

%f is a lot more expensive than it should be because we're not using the cached
NumberFormat (as we are for %d). Running the microbenchmark I added to the
bug (times in ms, on a Cortex A8):

                                old       new
      new Formatter %f         1732       811
      String.format %d          635       651
      String.format %f         1752       900
      reuse Formatter %f       1521       188
      Double.toString +         149       148
      Double.toString append     33        33
      StringBuilder.append      143       139

Bug: 1476

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