Pin Java bindings to 1.7

The android_library bazel rule currently enforces Java 7
6c1106b1a7/src/main/java/com/google/devtools/build/lib/bazel/rules/android/BazelAndroidSemantics.java (L73)

More generally, to enable broader use we pin the source and artifacts
to 1.7 till Java 8 gains wider adoption across Android.

tensorflow/java/build_defs.bzl

@@ -1,5 +1,14 @@
 # -*- Python -*-
 
+# Pin to Java 1.7 to ensure broader compatibility for the Java bindings on
+# Android. Note also that the android_library bazel rule currently enforces
+# java 7
+# https://github.com/bazelbuild/bazel/blob/6c1106b1a721516d3b3db54d2e1c31b44a76fbb1/src/main/java/com/google/devtools/build/lib/bazel/rules/android/BazelAndroidSemantics.java#L73
+
+JAVA_VERSION_OPTS = [
+    "-source 7 -target 7",
+]
+
 # A more robust set of lint and errorprone checks when building
 # Java source to improve code consistency.
 
@@ -151,4 +160,4 @@ EP_DISABLED_CHECKS = [
 
 EP_OPTS = EP_ENABLED_WARNINGS + EP_DISABLED_CHECKS
 
-JAVACOPTS = XLINT_OPTS + EP_OPTS
+JAVACOPTS = JAVA_VERSION_OPTS + XLINT_OPTS + EP_OPTS

tensorflow/java/maven/libtensorflow/pom.xml

@@ -19,8 +19,8 @@
         <artifactId>maven-compiler-plugin</artifactId>
         <version>3.6.1</version>
         <configuration>
-          <source>1.8</source>
+          <source>1.7</source>
-          <target>1.8</target>
+          <target>1.7</target>
         </configuration>
       </plugin>
       <plugin>

tensorflow/java/maven/proto/pom.xml

@@ -27,8 +27,8 @@
         <artifactId>maven-compiler-plugin</artifactId>
         <version>3.6.1</version>
         <configuration>
-          <source>1.8</source>
+          <source>1.7</source>
-          <target>1.8</target>
+          <target>1.7</target>
         </configuration>
       </plugin>
       <plugin>