Revert "Revert "Make OpenJDK 8 the default""

Reapply the JDK 8 switch in internal builds.

This reverts commit 85bddfdedf.

envsetup.sh

@@ -1426,7 +1426,8 @@ function set_java_home() {
     fi
 
     if [ ! "$JAVA_HOME" ]; then
-      if [ ! "$EXPERIMENTAL_USE_JAVA8" ]; then
+      if [ -n "$LEGACY_USE_JAVA7" ]; then
+        echo Warning: Support for JDK 7 will be dropped. Switch to JDK 8.
         case `uname -s` in
             Darwin)
                 export JAVA_HOME=$(/usr/libexec/java_home -v 1.7)