Set os_type=android on ToTAndroidCFI.

This causes the bot to receive the correct sharding settings
for Android bots.

Bug: 849825
Change-Id: I14b45cc214ce5eb13000460911780bf1c256a4dc
Reviewed-on: https://chromium-review.googlesource.com/1087780Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564705}

testing/buildbot/waterfalls.pyl

@@ -619,6 +619,7 @@
             },
           ],
         },
+        'os_type': 'android',
         'test_suites': {
           'gtest_tests': 'chromium_android_gtests',
         },