Skip only Android states, not nodes, on Travis

Instead of skipping all node types that include Android, use the pillar
to skip Android states on Travis.

Providing 'travis: True' in the pillar for a node will enable skipping
Android states; the key can simply be omitted if not on Travis, and Salt
will fall back to running Android states by default.

.travis.yml

@@ -13,13 +13,12 @@ matrix:
       env: SALT_NODE_ID=servo-mac1
     - os: osx
       env: SALT_NODE_ID=servo-macpro1
-    # Android is flaky on Travis CI, so skip android-related nodes
-    #- os: linux
-    #  env: SALT_NODE_ID=servo-linux-android1
-    #- os: linux
-    #  env: SALT_NODE_ID=servo-head
     - os: linux
       env: SALT_NODE_ID=linux1
+    - os: linux
+      env: SALT_NODE_ID=servo-linux-android1
+    - os: linux
+      env: SALT_NODE_ID=servo-head
 
 before_install:
   - .travis/install_salt

.travis/test_pillars/top.sls

@@ -1,5 +1,6 @@
 base:
   '*':
+    - travis
     - buildbot.common
 
   'servo-master':

.travis/test_pillars/travis.sls

@@ -0,0 +1 @@
+travis: True

android-dependencies.sls

@@ -1,3 +1,6 @@
+# Android is flaky on Travis
+{% if not salt['pillar.get']('travis', False) %}
+
 android-dependencies:
   pkg.installed:
     - pkgs:
@@ -85,3 +88,5 @@ android-ndk-toolset-configuration:
     - user: servo
     - group: servo
     - mode: 0644
+
+{% endif %}