Enable String compression.

Test: m test-art-host (interpreter, JIT, AOT)
Test: m test-art-target on Nexus 6P (interpreter, JIT, AOT)
Test: Nexus 6P boots
Test: m ahat-test
Bug: 31040547

(cherry picked from commit 60d049e21528268e2dcfe36cf6902b1a07a60b93)

Change-Id: I21145eae7d76b12d018918c45877d125ade3e9e4
diff --git a/ojluni/src/main/java/java/lang/String.java b/ojluni/src/main/java/java/lang/String.java
index 2d985ca..115890e 100644
--- a/ojluni/src/main/java/java/lang/String.java
+++ b/ojluni/src/main/java/java/lang/String.java
@@ -536,7 +536,7 @@
      *          object.
      */
     public int length() {
-        final boolean STRING_COMPRESSION_ENABLED = false;
+        final boolean STRING_COMPRESSION_ENABLED = true;
         if (STRING_COMPRESSION_ENABLED) {
             // For the compression purposes (save the characters as 8-bit if all characters
             // are ASCII), the least significant bit of "count" is used as the compression flag.