Add a password strength meter to SetupActivity. Dev task #42.

briar-api/src/org/briarproject/api/crypto/PasswordStrengthEstimator.java

@@ -0,0 +1,16 @@
+package org.briarproject.api.crypto;
+
+public interface PasswordStrengthEstimator {
+
+	float NONE = 0;
+	float WEAK = 0.4f;
+	float QUITE_WEAK = 0.6f;
+	float QUITE_STRONG = 0.8f;
+	float STRONG = 1;
+
+	/**
+	 * Returns an estimate between 0 (weakest) and 1 (strongest), inclusive,
+	 * of the strength of the given password.
+	 */
+	float estimateStrength(char[] password);
+}