Revert "Make OpenJDK 8 the default"

Reverting until all continuous builds that cover branches downstream of master can be updated to use JDK 8.

This reverts commit 1bd16b67c4b05f9df1f6d9ef85365929df0c34ef.

Change-Id: Icb0daa440a6286e07d3cfd9ff8b082b4154ebc21
diff --git a/envsetup.sh b/envsetup.sh
index 61b9ff0..cb3dc10 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -1426,8 +1426,10 @@
     fi
 
     if [ ! "$JAVA_HOME" ]; then
-      if [ -n "$LEGACY_USE_JAVA7" ]; then
-        echo Warning: Support for JDK 7 will be dropped. Switch to JDK 8.
+      if [ ! "$EXPERIMENTAL_USE_JAVA8" ]; then
+        echo JDK 8 will soon become the default. Please install it now.
+        echo Also: \"export EXPERIMENTAL_USE_JAVA8=true\"
+        echo See http://go/android-jdk8-discuss
         case `uname -s` in
             Darwin)
                 export JAVA_HOME=$(/usr/libexec/java_home -v 1.7)