Fixed two bugs with the KeepModifier with shrob.

- When using two rules with potential modifiers, make sure
  that the modifier variable was reset to NONE.
- Modifier should not be an enum since more than one modifier
  could be used.

Bug: 25413316
Bug: 25413069

(cherry picked from commit b5b9f53e94d5e3eb3fe3a5ff7d33664e2512de0c)

Change-Id: I6d6e373a98fc43653c661187ffafec48c3f0dddd
7 files changed