There are several ways to resolve your issue.
1. Add a prebuilt .apk
You don't have to put your source code to the AOSP tree.
You can just add your .apk file, put it either in packages/apps/YourApp, or vendor/yourname/packages/apps/YourApp, or even your_dir_name/packages/apps/YourApp, and create an Android.mk file for build system to determine your application.
Android.mk will look like:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := YourApplication # your .apk name
LOCAL_SRC_FILES := $(LOCAL_MODULE).apk
LOCAL_MODULE_CLASS := APPS
LOCAL_MODULE_SUFFIX := $(COMMON_ANDROID_PACKAGE_SUFFIX)
include $(BUILD_PREBUILT)
Pros: you can build your project with gradle.
2. Add source code to AOSP
If you still want to place your source code to packages/apps and build it there, you can put a ConstrainsLayout to your project's libs/ directory and add to your Android.mk something like:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# List of static libraries to include in the package
LOCAL_STATIC_JAVA_LIBRARIES := constraint-layout
# Build all java files in the java subdirectory
LOCAL_SRC_FILES := $(call all-subdir-java-files)
# Name of the APK
LOCAL_PACKAGE_NAME := YourApplication
# Tell it to build an APK
include $(BUILD_PACKAGE)
In case you will not get it work (I haven't met this issue, but he did):
LOCAL_STATIC_JAVA_LIBRARIES := libconstraint-layout
include $(BUILD_PACKAGE)
Other stuff, and finally
include $(CLEAR_VARS)
LOCAL_PREBUILT_STATIC_JAVA_LIBRARIES := libconstraint-layout:libs/constraint-layout.aar
Cons: You will have to build your code either with make by mma or mm -B, or to have a gradle as your second build system for development. The second option will work, but to establish a full build and to have your .apk built in out/ directory you will have to build it with make.
3. Adding a ConstraintLayout
In case you want to have several applications, which use a constraint layout, you can add it as a new library module as precompiled .aar.
Can be somewhere in 'vendor/yourname/libs' or 'your_dir_name/libs' respectively.
It is similar to adding a prebuilt .apk:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := constraint-layout
LOCAL_SRC_FILES := $(LOCAL_MODULE).aar
LOCAL_MODULE_SUFFIX := .aar
include $(BUILD_PREBUILT)
After that, in your application's Android.mk you will have to add:
LOCAL_STATIC_JAVA_LIBRARIES := constraint-layout
Alternatively, you can add a ConstraintLayout's .aar to the prebuilds/ as it eventually will be there someday.
There is a good topic about Android.mk: https://wladimir-tm4pda.github.io/porting/build_cookbook.html
