Fixed page_source() for newer Android versions.

Fix for https://github.com/appium/appium/issues/675.

On my Nexus 5 (4.4.2), the hierarchy file name is different from what page_source() expects.
See:
http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.4.2_r1/com/android/uiautomator/core/UiDevice.java?av=f#769
So the file path will be ``/data/local/tmp/local/tmp/dump.xml``.

With this fix we will try to look for a list of possible dump files before giving up and returning an empty string. It also makes sure that ``/data/local/tmp/local/tmp`` exists otherwise dumpWindowHierarchy will fail.