Revert "Revert "Make OpenJDK 8 the default""

Reapply the switch to JDK 8 on aosp/master.

This reverts commit 85bddfdedffb425d85657314591a57191113806b.

Change-Id: If2e8733ba78b007d37914c0512cd6be1bd4c1e69
diff --git a/envsetup.sh b/envsetup.sh
index f266f1a..61b9ff0 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -1426,7 +1426,8 @@
     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)