Avoiding flush on-change sensors at subscription

Avoiding flush on-change sensors at subscription

Initial sensor flush at subscription is a mechanism to avoid sensors
to get stale samples before subscription happens. However, there is a
slight chance that a most recent sample will be lost during the flush
process. This is OK for continuous sensors but problematic in
on-change sensor as on-change event does not come continuously and
a lost event can cause inconsistent state in client. Flush at
subscription of on-change sensor is disabled in this CL to avoid new
important on-change event to be discarded during the initial flush
process.

Bugs: b/24647069
      b/25241873
      b/24804819

Change-Id: Ibda099c6b9f5fb6e200f13cf13a850b0026e9e7c