media.log: improve histogram appearance

removed whitespace
fix histogram column appearance, removed y-axis labels
added axis labels and removed unnecessary characters in columns
minor code cleanup

New appearance:
[446203.725] Histogram flush - FastMixer: 6F660152
  Occurrences
      1  19  15   1   1

          |
          |   |
          |   |
          |   |
      |   |   |   |   |
   ------------------------
      0   3   4   6  29    ms

Old appearance:
[54.234] Histogram flush - AudioOut_D:
Histogram 33640BF1
            [ 1][ 1][ 1][ 3][54][69][ 1][ 2][ 1]
        64|                      []
        32|                  []  []
        16|                  []  []
         8|                  []  []
         4|                  []  []
         2|______________[]__[]__[]______[]____
              4   5   6   8   9  10  11  13  15

Test: dumpsys media.log
Change-Id: Iece8feec66db1cef66db4cb12d4db3f7e433b7d5