Use a newer version of Mockito, as necessary for Java 9.

Apparently 2.8.40 is out, but currently we're internally upgrading to 2.7.19, so that seems like the safest thing to target. (But I'm sure we'll get to 2.8.40 or later eventually, so maybe I'm wasting time bothering with 2.7.19....)

(part of #2571)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158745717

Author: cpovirk

android/pom.xml

@@ -238,7 +238,7 @@
       <dependency>
         <groupId>org.mockito</groupId>
         <artifactId>mockito-core</artifactId>
-        <version>2.1.0</version>
+        <version>2.7.19</version>
         <scope>test</scope>
       </dependency>
       <dependency>

pom.xml

@@ -234,7 +234,7 @@
       <dependency>
         <groupId>org.mockito</groupId>
         <artifactId>mockito-core</artifactId>
-        <version>2.1.0</version>
+        <version>2.7.19</version>
         <scope>test</scope>
       </dependency>
       <dependency>