ImportKeys: Use cache for Highlighter

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/LruCache.java

@@ -0,0 +1,22 @@
+package org.sufficientlysecure.keychain.util;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+
+// Source: http://stackoverflow.com/a/1953516
+public class LruCache<A, B> extends LinkedHashMap<A, B> {
+
+    private final int maxEntries;
+
+    public LruCache(final int maxEntries) {
+        super(maxEntries + 1, 1.0f, true);
+        this.maxEntries = maxEntries;
+    }
+
+    @Override
+    protected boolean removeEldestEntry(final Map.Entry<A, B> eldest) {
+        return super.size() > maxEntries;
+    }
+
+}