Restore old behavior for Android in fileutils.cc

From r7014 the Android APK tests are designed to be
build from a standalone WebRTC checkout instead of a
Chromium checkout. Because of that, the special handling
for both cases can be removed.

I also don't think we need to use the
base::android::GetExternalStorageDirectory() method since
all devices has a symlink at /sdcard that points
to /storage/emulated/legacy on the Android device.

This essentially reverts the changes in
https://webrtc-codereview.appspot.com/1754005/
plus some minor changes.

BUG=webrtc:3741
TEST=Locally running test_support_unittests APK test on an
Android device using:
CHECKOUT_SOURCE_ROOT=`pwd` build/android/test_runner.py gtest -s test_support_unittests --verbose --isolate-file-path=webrtc/test/test_support_unittests.isolate
R=henrike@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15329004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7632 4adac7df-926f-26a2-2b94-8c16560cd09d