Fix robolectric tests when running with gradle

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=186291489

ISSUE_TEMPLATE

@@ -1,3 +1,5 @@
+*** ISSUES THAT IGNORE THIS TEMPLATE WILL BE CLOSED WITHOUT INVESTIGATION ***
+
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 - Search existing issues, including issues that are closed.

constants.gradle

@@ -26,8 +26,8 @@ project.ext {
     dexmakerVersion = '1.2'
     mockitoVersion = '1.9.5'
     junitVersion = '4.12'
-    truthVersion = '0.35'
-    robolectricVersion = '3.4.2'
+    truthVersion = '0.39'
+    robolectricVersion = '3.7.1'
     releaseVersion = '2.7.0'
     modulePrefix = ':'
     if (gradle.ext.has('exoplayerModulePrefix')) {

library/core/src/test/resources/robolectric.properties

@@ -0,0 +1 @@
+manifest=src/test/AndroidManifest.xml