Detach pollThread only if it is joinable

sensor HAL is not required for Android TV. Hence, ISensors returned
from getService() was null and pollThread was not associated with
any thread of execution.

Bug: 120910004
Test: run vts -m VtsHalSensorsV1_0Target
Change-Id: If16af7313ff7bb22e5ca16c03a0fee0b8d733335