Android test runner: support negative gtest filter syntax.

Using --gtest_filter=-AudioInputTest.* currently results in no tests being
run, instead of "all tests except for AudioInputTest".

R=skyostil@chromium.org,cjhopman@chromium.org

Review URL: https://codereview.chromium.org/1235713003

Cr-Original-Commit-Position: refs/heads/master@{#338396}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: db9824bb2ffd51cc023e19077749ffe777887223

util/lib/common/unittest_util.py

@@ -129,7 +129,9 @@ def FilterTestNames(all_tests, gtest_filter):
     Filtered subset of the given list of test names.
   """
   pattern_groups = gtest_filter.split('-')
-  positive_patterns = pattern_groups[0].split(':')
+  positive_patterns = ['*']
+  if pattern_groups[0]:
+    positive_patterns = pattern_groups[0].split(':')
   negative_patterns = None
   if len(pattern_groups) > 1:
     negative_patterns = pattern_groups[1].split(':')