Correct FixedPointParameter display precision

BZ: 176178

Explain the precision computation (m * log10(2)):

  For a Qn.m number, the step between each storable number is 2^(-m).
  Hence, on a decimal representation, the Dth digit after the decimal
  point can take all possible values (1..9) - meaning that it is
  significant - only if

          2^(-m) <= 10^(-D)
              -m <= log2(10^(-D))
              -m <= log10(10^(-D)) / log10(2)
              -m <= -D / log10(2)
    m * log10(2) >= D

  Conversly, the Dth digit can be represented if

    D <= m * log10(2)

We add 1 to the precision in order to display the digit right after the last
significant digit.  This will lead to oddities such as:

    $ setParameter /Test/test/f32_Q8.23 0.1234569
    $ getParameter /Test/test/f32_Q8.23
    > 0.1234570

but it will avoid modifying raw values when converting from string to Q-format
and back to string.

Also, use std::setprecision() on the stream instead of manually truncating the
displayed number.

Change-Id: Ief2a7daabf4505ae4312e79036b0374f53368cac
Signed-off-by: David Wagner <david.wagner@intel.com>

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