fix: repair the Android build (#905)

Rather than hardcoding the NDK version inside a script, encode it
as a file in the filesystem, which is easier to share.

Make sure we use the desired NDK by setting environment variables.

Use `-androidabi 21`, which:

1. is what rclone did: https://github.com/rclone/rclone/commit/8390ba4ca9f18fea546ecdfdf8f86aed5c36030f

2. is the minimum ABI used by probe-android: https://github.com/ooni/probe-android/blob/994651be52ff75ee3a00dba499ec7c7b40ab25c1/app/build.gradle#L10

Part of https://github.com/ooni/probe/issues/2130

MOBILE/gomobile

@@ -42,6 +42,14 @@ else
 	OONI_PSIPHON_TAGS=""
 fi
 
+MAYBE_ANDROID_API=""
+if [[ $TARGET == "android" ]]; then
+	ANDROID_HOME=$(./MOBILE/android/home)
+	NDK_VERSION=$(cat NDKVERSION)
+	ANDROID_NDK_HOME=$ANDROID_HOME/ndk/$NDK_VERSION
+	MAYBE_ANDROID_API="-androidapi 21"
+fi
+
 PRODUCT=$(basename $PACKAGE)
 
 function cleanup() {
@@ -51,11 +59,17 @@ function cleanup() {
 trap cleanup EXIT
 
 set -x
+
+# for documenting the environ variables used by the build
+export ANDROID_HOME=$ANDROID_HOME
+export ANDROID_NDK_HOME=$ANDROID_NDK_HOME
+
 go install golang.org/x/mobile/cmd/gomobile@latest
 $(go env GOPATH)/bin/gomobile init
 # Adding gomobile to go.mod as documented by golang.org/wiki/Mobile
 go get -d golang.org/x/mobile/cmd/gomobile
 
 $(go env GOPATH)/bin/gomobile bind -target $TARGET \
-	-o ./MOBILE/$TARGET/$PRODUCT.$EXT -tags="$OONI_PSIPHON_TAGS" \
+	-o ./MOBILE/$TARGET/$PRODUCT.$EXT $MAYBE_ANDROID_API \
+	-tags="$OONI_PSIPHON_TAGS" \
 	-ldflags '-s -w' ${GOLANG_EXTRA_FLAGS:-} $PACKAGE