It seems as though Google is preparing to introduce a new feature to Android which will provide feedback on the current battery levels of connected Bluetooth devices. This is already a feature available on select handsets, and through various third-party ROMs, however, it now looks as though it could be one that is included by default on stock Android. This is due to the feature showing up in the Android Open Source Project (AOSP), largely confirming that it is a feature being worked on and one which will likely be implemented in a future update.
The downside however is the timing. As the fourth developer preview (which is generally considered to be the finished developer version) has already been released by Google, it seems highly unlikely that this feature will be included in the final consumer-ready version of Android O when it is released. Which likely means it will be released at a later time, with one possibility being that it will arrive as part of Android O's next major incremental update.
Of course, most Bluetooth devices do now come with their own app and these apps usually provide a way for users to view the current battery status of a connected device. So while this is not exactly adding new features, what it will do is provide an easier and quicker way to view the battery life of connected Bluetooth devices. Likewise, in spite of this being a feature that is already available on a number of Android devices, this will be of particular relevance to those who prefer to stick with a more stock-like Android experience. It is also worth noting that the code suggests there could be a number of ways in which the feature can be implemented. So in addition to displaying the battery level as a battery, it will likely be possible that numerical values could be used instead. Which also might mean that the user will be able to choose (through the device's settings ) the way in which the battery levels of connected Bluetooth devices show up – much like Android device owners currently can with the device's battery in general.