This CL adds unit tests for MediaCodecLoop that compile and run
anywhere.  We do this, so that we don't accidentally take any
dependencies on the android runtime, e.g., BuildInfo.  It does
this with new build targets.

For example, we know that media_codec_loop.cc does not depend on
JNI to run, and doesn't care if (for example) MediaCodec is broken
today on the particular device on which the tests run.

Moved MediaCodecBridge and MediaCodecLoop to android/anywhere, which
builds anywhere.  For example, building media_unittests on linux will
include tests for MediaCodecLoop.

Note that building non-unittest targets on non-android platforms will
not include extra code.

BUG=632401

Review-Url: https://codereview.chromium.org/2132653002
Cr-Commit-Position: refs/heads/master@{#411075}