Android development is divided into few categories. One is source-built development, which includes creating applications or custom device trees. Another category involves modifying system applications that are closed source. These applications can be obtained from the device with ADB command or by extracting the image file of the update. It’s quite simple on Linux, as few commands can do the trick, but situation gets more complicated when Windows is your desktop platform of choice.
If you are planning to extract an image file, you should try a nice graphical editors, like the one shared by XDA Senior Member dbzfanatic. His tool allows you to easily unpack IMG files. The app also allows you to view some files as text or hex, which might be important to understand which parts need to be changed. After extraction, files can be decompiled, changed with APKTool, and packed back as a custom ROM with your own improvements.
You can make your way to the utility thread to download the latest version._________
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