As announced Qt team ported Qt libraries to work on mobile system also but with some limitations. The most "important" limitation (in my opinion) is the choice to not include QWebKit (or QtWebEngine) component into mobile system library set. As replacement of this lack Qt team provide the QtWebView component that is a wrapper around the system native web browser engine.

If you want to launch an Android app from shell command line terminal there is a simply command allow to do that but you have to know some info about the app you want to execute. The command is named am and is basically a command line interface to the system ActivityManager.

If you subclass a widget control and try to repaint the content by overriding the paintEvent() method you could experiment, for some specific type of Qt widgets, the error "Painter not active". This error come up during program execution when try to create the QPainter object for draw the widget content. The problem usually came from the use of the wrong QPaintDevice pointer. Some Qt widgets, like for example QGraphicsView, have an additional "internal" paint area usually called viewport that need to be used for paint instead of the main object. Translating this explanation into code we can use as example the subclass of the QGraphicsView as follow:

void MyGraphicsView::paintEvent(QPaintEvent * event)
{
    QPainter painter(this);

    // Here draw the content
}

This code will show up the "Painter not active" error because for this specific widget the internal viewport have to be used. The correct code will be the following:

void MyGraphicsView::paintEvent(QPaintEvent * event)
{
    QPainter painter(viewport());

    // Here draw the content
}

Through the call viewport() we get the working QPaintDevice pointer to use for paint widget area.
If you are developing an Android app and need the emulator to access Internet you can get the result in case of your network is under a proxy. The emulator interface doesn't have some GUI dialog allow to insert proxy params but there is a way to set these params by command line. This mean you have to manually launch the emulator instead of have it automatically started by the IDE. The command line format is the following:

emulator.exe @[emulator image name] -http-proxy http://[username]:[password]@[proxy IP]:[proxy port]

This example is valid under Windows, in case of Linux simply remove the .exe extension from emulator executable file name. Let's make some practical examples just for explain better the use. In this example we suppose to have an emulator android image called "Image test" (remember in case you have an emulator image name containing spaces you have to change it with the character '_'). We'll use as proxy IP the classic 192.168.0.1 and port 8080. As username we'll use 1234 and password 5678 (don't use it at home or in office ^_^). Using these data we'll have:

emulator.exe @Image_test -http-proxy http://1234:5678@192.168.0.1:8080

In case your proxy doesn't require authentication you simply have:

emulator.exe @Image_test -http-proxy http://192.168.0.1:8080

Launch the emulator using these command line will allow some app like standard browser (and then WebView component too) to correctly access Internet through the proxy in a transparent way. Hope this help.
Qt Creator recently introduced a new way for develop android application. Basically is now possible to select use Gradle instead of apache-ant for final APK creation. Gradle is going to be the new "standard" than the choice of Qt team look perfectly reasonable. However if you are developing in a network under a proxy you could experiment some problem using Gradle.