[Tizen] Move platform sensor initialization out from browser UI thread.

Cold start time of sample Calulator application increases to ~1080 ms on
Tizen VTC-1010 at the moment. One bottleneck is it takes long period to
initalize platform sensors inside browser main thread, which blocks native
window display and starting render message loop.

The CL moves the platform sensors initialization outside of browser main
thread. The local test result shows it shortens ~200ms of cold start
time.

BUG=XWALK-2520
for (auto observer : observers_)
     observer->OnSensorConnected();