Reduce EntropyMixer seed size from 4096 to 512 bytes.

The suggested seed size from Linux drivers/char/random.c is 512 bytes
rather than the 4096 bytes currently used.

Change-Id: I3dd55105f0a95ce9cda34b4cf08987b8a28c4ac9
diff --git a/services/java/com/android/server/RandomBlock.java b/services/java/com/android/server/RandomBlock.java
index e5d73015..6d6d901 100644
--- a/services/java/com/android/server/RandomBlock.java
+++ b/services/java/com/android/server/RandomBlock.java
@@ -27,13 +27,13 @@
 import java.io.RandomAccessFile;
 
 /**
- * A 4k block of random {@code byte}s.
+ * A block of 512 random {@code byte}s.
  */
 class RandomBlock {
 
     private static final String TAG = "RandomBlock";
     private static final boolean DEBUG = false;
-    private static final int BLOCK_SIZE = 4096;
+    private static final int BLOCK_SIZE = 512;
     private byte[] block = new byte[BLOCK_SIZE];
 
     private RandomBlock() { }