Fdroidserver building process

This is a detailed note about how the fdroidserver builds an app based on the Builds blocks in the metadata.

1. NDK is checked. If ndk is set, fdroidserver will check if NDK is installed. If not, build will failed directly.
2. Env variables are set: ANDROID_HOME, ANDROID_SDK and ANDROID_SDK_ROOT are set for Android SDK. JAVA{version}_HOME is set for Java. ANDROID_NDK, NDK and ANDROID_NDK_HOME are set for Android NDK.
3. sudo: is executed. Please note that the Repo hasn't been cloned so that sudo is executed in the root directory of fdroiddata. Variables haven't been replaced yet so that $$xxx$$s do not work. After sudo:, the sudo binary will be removed.
4. Repo is cloned into build/{appID} and checkout to commit. If submodules:true then submodules will be initialised, recursively.
5. Path is changed to build/{appID}. If subdir: is set, the path will change to it.
6. init: is executed. Please note that $$xxx$$s are not available in init.
7. patch: is applied on build/{appID}.
8. srclibs: are cloned and prepared.
9. local.properties files are updated. sdk.dir and ndk.dir are set.
10. If gradle: is set, build.gradle/build.gradle.kts is processed.
11. Debuggable flags are removed.
12. forceversion: and forcevercode: are inserted.
13. Paths in rm: are removed.
14. Keysigning configs will be removed from build.gradle properties files. Please note this may break the build and needs to be fixed in prebuild.
15. prebuild: is executed.
16. If none of maven:, gradle:, buildozer: and output: is set, ant will be used by default and android update project will be executed. Because android has been removed from Android SDK, the build will fail.
17. The clean task is performed.
18. The source code is scanned for common problems. Binaries in scandelete: will be removed and in scanignore: will be ignored.
19. build: is executed.
20. buildjni: is executed.
21. maven/buildozer/gradle/ant is executed.
22. APK file in output path is picked and checked. Output path is calculated from build/{appID}/{output path}. If subdir: is set, build/{appID}/{subdir}/{output path}