Substantially improve Scrubber performance.

Previously, we were making a new copy of the entire source string after
every scrubbed substitution. In the case of our new, larger log files,
this was very slow. It's been changed so we only ever create one new
copy.

In practice, on a Moto E (2014), scrubbing a 1.5MB log went from
>4000ms to ~100ms.

src/org/thoughtcrime/securesms/logsubmit/util/Scrubber.java

@@ -47,17 +47,22 @@ public class Scrubber {
     String out = in;
     for (Pattern pattern : patterns) {
       Matcher       matcher       = pattern.matcher(out);
+      StringBuilder builder       = new StringBuilder();
+      int           lastEndingPos = 0;
+
       while (matcher.find()) {
+        builder.append(out.substring(lastEndingPos, matcher.start()));
 
-        StringBuilder builder = new StringBuilder(out.substring(0, matcher.start()));
         final String censored = matcher.group().substring(0,1)                                      +
                                 new String(new char[matcher.group().length()-3]).replace("\0", "*") +
                                 matcher.group().substring(matcher.group().length()-2);
         builder.append(censored);
-        builder.append(out.substring(matcher.end()));
+
+        lastEndingPos = matcher.end();
         Log.i(TAG, "replacing a match on /" + pattern.toString() + "/ => " + censored);
-        out = builder.toString();
       }
+      builder.append(out.substring(lastEndingPos));
+      out = builder.toString();
     }
     return out;
   }