As described in this answer, the AOSP apps don’t come as a gradle project, but are usually compiled in a system build (where they also have access to other dependencies and resources).

So building the source as part of a gradle project is not straight-forward, some adaptions are necessary.

I successfully got a subset of DeskClock (e.g. without Timer, Stopwatch and some further UI parts) to compile with gradle with the following steps:

• Clone the AOSP code
• In a new or existing AndroidStudio/Gradle project, create a new library module (if you want to use it as part of another app, otherwise create an app project)
  • Find information for minSdk etc. in the AOSP code’s AndroidManifest.xml
• Adapt the library’s AndroidManifest.xml according to the AOSP code’s AndroidManifest.xml (it should not necessarily be the same)
• Copy the classes/packages/resources from the AOSP code you need to your library module under src/main/ (into the respective subdirectories java, res etc.)
• Try to compile and repeatedly add the missing source files referenced from the copied sources
• Add missing library dependencies; they can be detected by unresolved imports, e.g. those of androix libraries
• Fix the sources for references which do not work in a normal app/library context:
  • Add the following to build.gradle.kts if the code references the BuildConfig class:

    buildFeatures {
        buildConfig = true
    }
    

  • Change BuildConfig.APPLICATION_ID to BuildConfig.LIBRARY_PACKAGE_NAME (in case you are using a library module)
  • (optional) Remove unused resources with the corresponding inspection
  • Fix unresolved references to resource values R.xxx
    • Some are Android platform resources, change the reference to android.R.xxx
    • Some are from libraries, e.g. androidx.preference.R.attr.dropdownPreferenceStyle
  • Change some const val which refer to resources but which fail to compile because they are not considered constant by removing the const modifier

Specific for DeskClock:

• You might have to introduce new Build.VERSION.SDK_INT checks in ClockContract, e.g. I needed to replace a usage of isOOrLater because it actually required O_MR1:

  val isOMR1OrLater: Boolean
        get() = Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1