Reintroduce the contract for random token to be non-negative

Clients like adb backup/restore confimation activity
depended on it.

Test: gts-tradefed run gts -m GtsBackupHostTestCases
Bug: 64042228
Bug: 63691912
Bug: 63695367
Change-Id: I09e348f7570bcdbad2d91e79f90a248b17c82940

services/backup/java/com/android/server/backup/BackupManagerService.java

@@ -769,7 +769,9 @@ public class BackupManagerService implements BackupManagerServiceInterface {
     // side unpredictability.
     @Override
     public int generateRandomIntegerToken() {
-        int token = mTokenGenerator.nextInt() & ~0xFF;
+        int token = mTokenGenerator.nextInt();
+        if (token < 0) token = -token;
+        token &= ~0xFF;
         token |= (mNextToken.incrementAndGet() & 0xFF);
         return token;
     }