AndroidVDA: reduce decode latency via opportunistic feed/drain of the HW codec.

CPU utilization remains untouched but the histogram of time between
AVDA::Decode() receiving a BitstreamBuffer and MediaCodec.queueInputBuffer()
taking it goes from this:

0    ------------------------------------------------------------------------O (644 = 11.1%)
1    ----------------------------------------------------------O               (518 = 9.0%) {11.1%}
2    ---------------------------------------------------------O                (506 = 8.8%) {20.1%}
3    --------------------------------------------------------O                 (500 = 8.6%) {28.8%}
4    -------------------------------------------------------O                  (489 = 8.5%) {37.5%}
5    ---------------------------------------------------------O                (509 = 8.8%) {46.0%}
6    ----------------------------------------------------------O               (515 = 8.9%) {54.8%}
7    ----------------------------------------------------------O               (515 = 8.9%) {63.7%}
8    --------------------------------------------------------------O           (1106 = 19.1%) {72.6%}
10   -------------------O                                                      (339 = 5.9%) {91.7%}
12   --O                                                                       (34 = 0.6%) {97.6%}

to this:

0   ------------------------------------------------------------------------O (2871 = 98.4%)
1   O                                                                         (12 = 0.4%) {98.4%}
2   O                                                                         (3 = 0.1%) {98.8%}
3   O                                                                         (1 = 0.0%) {98.9%}

(bucket names are milliseconds; the long tail of the before histogram goes out
to 190ms, the long tail of the "after" histogram goes out to 34ms)

Review URL: https://codereview.chromium.org/41923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230954 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230954