base android images on Ubuntu 20.04

the repo tool (all of its versions?) now depend on a python 3.6+ interpreter
Ubuntu 16.04 only has access to a python 3.5 interpreter which is too old and won't work
using a newer Ubuntu image shouldn't affect the minimum supported android version (*) because we are
building an android SDK from scratch and android doesn't depend on gliblc

(*) old Ubuntu images are used for the linux-gnu targets to ensure compatibility with old glibc
versions

Author: japaric

docker/Dockerfile.aarch64-linux-android

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:20.04
 
 COPY common.sh /
 RUN /common.sh

docker/android-ndk.sh

@@ -30,6 +30,7 @@ main() {
     pushd "${td}"
     curl --retry 3 -sSfL "${NDK_URL}" -O
     unzip -q android-ndk-*.zip
+    rm android-ndk-*.zip
     pushd android-ndk-*
     ./build/tools/make_standalone_toolchain.py \
       --install-dir /android-ndk \

docker/android-system.sh

@@ -17,6 +17,7 @@ main() {
         g++-multilib
         make
         python
+        python3
     )
 
     # fake java and javac, it is not necessary for what we build, but the build
@@ -53,6 +54,9 @@ EOF
     curl --retry 3 -sSfL https://storage.googleapis.com/git-repo-downloads/repo -O
     chmod +x repo
 
+    # the `repo` tool requires python3 so change the default python interpreter
+    ln -sf python3 /usr/bin/python
+
     # this is the minimum set of modules that are need to build bionic
     # this was created by trial and error
     ./repo init -u https://android.googlesource.com/platform/manifest -b android-5.0.0_r1
@@ -84,6 +88,10 @@ EOF
         ;;
     esac
 
+    # revert default python interpreter; the code we are going to build expects
+    # `/usr/bin/python` to be a python2 interpreter
+    ln -sf python2 /usr/bin/python
+
     # avoid build tests
     rm bionic/linker/tests/Android.mk bionic/tests/Android.mk bionic/benchmarks/Android.mk