Avoid letters 0,O,l,I for wifi names and passwords

app/src/main/java/org/briarproject/hotspot/StringUtils.java

@@ -7,18 +7,27 @@ class StringUtils {
 
 	private static final Random random = new SecureRandom();
 
+	private static String digits = "123456789"; // avoid 0
+	private static String letters = "abcdefghijkmnopqrstuvwxyz"; // avoid l
+	private static String LETTERS = "ABCDEFGHJKLMNPQRSTUVWXYZ"; // avoid I, O
+
 	public static String getRandomString(int length) {
 		char[] c = new char[length];
 		for (int i = 0; i < length; i++) {
 			if (random.nextBoolean()) {
-				c[i] = (char) ('0' + random.nextInt(10));
+				c[i] = random(digits);
 			} else if (random.nextBoolean()) {
-				c[i] = (char) ('a' + random.nextInt(26));
+				c[i] = random(letters);
 			} else {
-				c[i] = (char) ('A' + random.nextInt(26));
+				c[i] = random(LETTERS);
 			}
 		}
 		return new String(c);
 	}
 
+	private static char random(String universe)
+	{
+		return universe.charAt(random.nextInt(universe.length()));
+	}
+
 }