ShellSubscriber: Avoid rollover bug

We initialize 'mToken' instead of leaving it uninitialized, making
it much less likely to rollover.  In additional, we change our
check to inequality, to handle rollover if that were to happen.

There is the (theoretical) possibility of exactly 2^32 other
requests being claimed between our claim and our check.  It's
assumed that is essentially impossible and not a concern.

Test: TreeHugger
Bug: 150619687
Change-Id: Iee303e05082a6b3b31ed546bd62d3afe67c771d8