STPK-1429 Return as many available events as possible from sensor_poll

STPK-1429 Return as many available events as possible from sensor_poll

No longer return a single event when returning from sensor_poll. This
should minimize the trips between the Android sensor framework layers
and the HAL module itself. We can actually return up to one event per
sensor type, which should fit nicely with the buffer size that we get.

Issue: STPK-1429

Change-Id: Ie5f9974bee92380de4b121e005ace8261b77801e
Signed-off-by: Patrick Porlan <patrick.porlan@intel.com>