Fix assumptions in SensorsHidlTest.NoStaleEvents

This test was making a couple of false assumptions which were causing it
to fail. The fixes are related to the following assertions:
  1. One-shot sensors do not report an initial event.
  2. Special sensors may not report an initial event.
  2. Some on-change sensors may not report an initial event.
The test now only checks for a stale event if the sensor reports an
initial event consistently.

Bug: 138758242
Test: ran on C2 DVT; only fails due to an improperly configured sensor
Change-Id: I83f0cb2f6e878244f3d94ae77f64bb8ed2f78e0b