buildserver: use standardized paths for the NDKs

This should help with reproducibility since some tools like Python still
include the build paths in the binaries.
https://gitlab.com/fdroid/fdroidserver/-/merge_requests/919#note_578180986

The default ANDROID_SDK_ROOT base dir of /opt/android-sdk is hard-coded in
buildserver/Vagrantfile.  The $ANDROID_HOME/ndk subdir is where Android
Studio will install the NDK into versioned subdirs.
https://developer.android.com/studio/projects/configure-agp-ndk#agp_version_41

buildserver/Vagrantfile

@@ -84,7 +84,7 @@ Vagrant.configure("2") do |config|
     args: [configfile['debian_mirror']]
   config.vm.provision "shell", path: "provision-android-sdk"
   config.vm.provision "shell", path: "provision-android-ndk",
-    args: ["/home/vagrant/android-ndk"]
+    args: ["/opt/android-sdk/ndk"]
   config.vm.provision "shell", path: "provision-gradle"
 
 end

buildserver/config.buildserver.yml

@@ -1,8 +1,8 @@
 sdk_path: /opt/android-sdk
 ndk_paths:
-  r10e: /home/vagrant/android-ndk/r10e
-  r21e: /home/vagrant/android-ndk/r21e
-  r22b: /home/vagrant/android-ndk/r22b
+  r10e: /opt/android-sdk/ndk/r10e
+  r21e: /opt/android-sdk/ndk/21.4.7075529
+  r22b: /opt/android-sdk/ndk/22.0.7026061
 
 java_paths:
   8: /usr/lib/jvm/java-8-openjdk-amd64

buildserver/provision-android-ndk

@@ -18,9 +18,15 @@ fi
 for version in r21e r22b; do
     if [ ! -e ${NDK_BASE}/${version} ]; then
         unzip /vagrant/cache/android-ndk-${version}-linux-x86_64.zip > /dev/null
-        mv android-ndk-${version} ${version}
+        mv android-ndk-${version} \
+	   `sed -En 's,^Pkg.Revision *= *(.+),\1,p' android-ndk-${version}/source.properties`
     fi
 done
 
+# allow gradle/etc to install missing NDK versions
+chgrp vagrant $NDK_BASE
+chmod g+w $NDK_BASE
+
+# ensure all users can read and execute the NDK
 chmod -R a+rX $NDK_BASE/
 find $NDK_BASE/ -type f -executable -print0 | xargs -0 chmod a+x