Only use androidAPILevel, not androidAPIVersion

For the moment, default to using API level 19, and glob to
find the tools matching that API level (e.g. build-tools/19*/aapt);
if multiple matching files are found, use the last one in the list
(which is sorted alphabetically).
When looking for files by API level, sort by version

When we're looking for (for example) aapt, which may occur
in two or more directories, sort the results by the
version number which is implicit in the parent directories of
the found aapt binaries.

For example, if we have 19.0.0 and 19.0.1 versions of aapt,
we want the 19.0.1 version, so we need to sort all the matching
19* versions and return the latest one.
Env should sort results returned by Finder

The results of a file search (e.g. for aapt) should be returned
as plain arrays (Android SDK agnostic) to Env; then Env should
take responsibility for choosing the "latest" matching file.

This is done by passing a filter function to findFiles(), which
optionally selects a single item from the array of files
returned (either by glob or by guessing directories). The
single returned path can then be returned as the location of the
desired "piece".

findDirectories() also now returns multiple matching directories,
and the last of them is selected as the desired "piece".

This allows all code which is Android-specific to stay in Env and
not have to go into Finder.
Search platforms/ directory to set default Android API level

If androidAPILevel is not explicitly set in config, look
at the platforms/android-* directories and choose the
last one found after alpha sorting.
[Finder] findDirectories() should reject with tried paths

On failure, findDirectories() should reject the promise with
an array of paths tried, for consistency with findFiles().
Sort Android directories by basename only

On Windows, some paths were being returned with forward
slashes for path separators, and others with double back
slash, which caused the sort to be incorrect.

Just sort by the basename of the paths under platforms/
(the android-* part) to work around this.
Safely fall back if no platforms/ directories
Ensure that file separators are correctly fixed
Merge pull request #34 from crosswalk-project/infer-android-api-version-26

Infer android api version