Fix minor time bug in ConsentCallback::getElapsedTimeMs

Previously, this function was actually returning a value in nanoseconds,
which made any comparisons with milliseconds invalid. This in turn
caused the dumpstate routine to abort too early for bug reports that
finish before the default timeout elapses (e.g. wifi and telephony
report modes).

Fix: 147153946
Test: manual, ensure that an empty report still waits for 30 seconds
  (USER_CONSENT_TIMEOUT_MS) before cancelling the consent dialog

Change-Id: I2b610d3a32148f706e0b2fc2e330a879795fc8ad