RESTRICT AUTOMERGE: Always use safe labels

This change combines two changes below:

Change 3998795:
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]

Change 3998735:
For display purposes, we should always be using safe labels. This
prevents excessively lonVg labels from preventing app uninstallation.

Bug: 62537081
Test: Manual. Install APK from bug and see that it can be uninstalled
Change-Id: I983be82bcd0f496e3665e1cf55d02a278bc86fcf