Set Qt Quick Controls 2 default style

We have set Qt Quick Controls 2's style in each application's main.cpp
QQuickStyle::setStyle("AGL");

This patch set the style globally with an environment variable as this
style is provided by the platform.
https://doc.qt.io/qt-5/qtquickcontrols2-styles.html#environment-variable

Change-Id: I5a8ee91cac8ad665ba0f7f22ffd79cab3a50f9a2
Signed-off-by: Tasuku Suzuki <tasuku.suzuki@qt.io>