Fix IndexOutOfBoundsException while silent provisioning check

- If hostpot off, we receive result through HotspotOffReceiver.
  Sometimes, during provisioning, we're used to receive this.
  In this case, we don't care.
  After provisioning, we'll receive the intent again.

- In stress test,
  index is sometimes invalid, equals to mCurrentTethers.size().
  So, when provisioning, We check whether index is valid or not.

BUG=23528220
Change-Id: I70f35e045042c6c81b1db03e2a44cd41d3e7437f