Make safe label more safe

* limit the absolute maximum size of the label to 50000 characters
[which is probably far more than necessary, but, can be dialed down]

* use a string buffer while processing the string [instead of creating
multiple string objects]

Bug: 62537081
Test: Manual. Install APK in bug and see that it can be uninstalled
Change-Id: Ibf63c2691ad7438a123e92110d95b1f50050f8b1
Merged-In: Ibf63c2691ad7438a123e92110d95b1f50050f8b1