Android: Update Android Studio canary version

Canary has updated to alpha 4.

BUG=620034

Change-Id: Ic8ded85e9a06dd7a4a610145ade1df2a027a60e9
Reviewed-on: https://chromium-review.googlesource.com/847938Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526979}

build/android/gradle/root.jinja

@@ -12,7 +12,7 @@ buildscript {
     }
     dependencies {
 {% if canary %}
-        classpath "com.android.tools.build:gradle:3.1.0-alpha01"
+        classpath "com.android.tools.build:gradle:3.1.0-alpha04"
 {% else %}
         classpath "com.android.tools.build:gradle:3.0.0"
 {% endif %}