Update subscription logic for VHAL ref impl.

Update the implementation for subscription logic. Add clearer
documentation for sampleRate and timestamp behavior. The sampleRate
specified in subscribeOptions is just a guidance to tell VHAL what
the polling rate could be. For timestamp, the timestamp returned
for each property must be the timestamp when that property is
updated, not when the property is retrieved.

Test: atest FakeVehicleHardwareTest
Bug: 225191802, 226000926
Change-Id: I1e886133258236eedfa7fcffe5c4fb49aead4f6f