maven_downloader.py: Use expanduser instead of HOME env

"HOME" doesn't always exist as a envvar
https://stackoverflow.com/questions/4028904/how-to-get-the-home-directory-in-python

Bug: 869249
TBR: jbudorick
Change-Id: I13f381d12ac75f3ec45b9736b980b30e32e602af
Reviewed-on: https://chromium-review.googlesource.com/1157648Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580408}

build/android/pylib/utils/maven_downloader.py

@@ -36,7 +36,8 @@ class MavenDownloader(object):
   _REMOTE_REPO = 'https://maven.google.com'
 
   # Default Maven repository.
-  _DEFAULT_REPO_PATH = os.path.join(os.getenv('HOME'), '.m2', 'repository')
+  _DEFAULT_REPO_PATH = os.path.join(
+      os.path.expanduser('~'), '.m2', 'repository')
 
   def __init__(self, debug=False):
     self._repo_path = MavenDownloader._DEFAULT_REPO_PATH