Set last probing time to always evaluate stall

isDataStall() will skip the stall check if NetworkMonitor sent
probe within the minimal evaluation time to prevent sending
probing too frequently. In differnent test targets, the probing
result may come back just before test tries to check wether
it's stall or not. It may result in the unexpected result.
Set the last probing time to always evaluate it since the test
does not try to exam that part.

Bug: 185082309
Test: atest NetworkMonitorTest in forrest for 20 sets
Change-Id: I52a215e9c5be2c0f069ca24f800d4d616bb437d8