Add a second thread for sensor service.

HIDL implementation defaults to using a single thread for a service.
This blocks on sensorservice poll. A minimum of two threads are needed
to allow for any meaningful control.

Bug: 35067266
Test: Verify sensors stream in both passthrough and binderized modes.
Change-Id: I44ccdb6db253ca9eb9903b51e05c1893945e297e