cargo-apk: Use min_sdk_version to select compiler target

According to [1] minSdkVersion is used to determine the compiler target
and ultimately limit what API is available at compile-time.  This is
for the most part because using/linking newer API will result in runtime
linker errors on these older platforms.

In other words, using the target sdk version which (as of writing, but
that will change in the near future too) defaults to the highest version
supported by the detected SDK makes it impossible to detect
compatibility issues (symbols that are unavailable) with your app on
older Android APIs despite setting min_sdk_version.

[1]: https://developer.android.com/ndk/guides/sdk-versions#minsdkversion

cargo-apk/src/apk.rs

@@ -79,13 +79,7 @@ impl<'a> ApkBuilder<'a> {
     pub fn check(&self) -> Result<(), Error> {
         for target in &self.build_targets {
             let triple = target.rust_triple();
-            let target_sdk_version = self
-                .manifest
-                .android_manifest
-                .sdk
-                .target_sdk_version
-                .unwrap();
-            let mut cargo = cargo_ndk(&self.ndk, *target, target_sdk_version)?;
+            let mut cargo = cargo_ndk(&self.ndk, *target, self.min_sdk_version())?;
             cargo.arg("check");
             if self.cmd.target().is_none() {
                 cargo.arg("--target").arg(triple);
@@ -158,9 +152,7 @@ impl<'a> ApkBuilder<'a> {
                 .join(artifact)
                 .join(artifact.file_name(CrateType::Cdylib, triple));
 
-            let target_sdk_version = config.manifest.sdk.target_sdk_version.unwrap();
-
-            let mut cargo = cargo_ndk(&config.ndk, *target, target_sdk_version)?;
+            let mut cargo = cargo_ndk(&config.ndk, *target, self.min_sdk_version())?;
             cargo.arg("rustc");
             if self.cmd.target().is_none() {
                 cargo.arg("--target").arg(triple);
@@ -233,19 +225,27 @@ impl<'a> ApkBuilder<'a> {
 
     pub fn default(&self) -> Result<(), Error> {
         let ndk = Ndk::from_env()?;
-        let target_sdk_version = self
-            .manifest
-            .android_manifest
-            .sdk
-            .target_sdk_version
-            .unwrap_or_else(|| ndk.default_platform());
         for target in &self.build_targets {
-            let mut cargo = cargo_ndk(&ndk, *target, target_sdk_version)?;
+            let mut cargo = cargo_ndk(&ndk, *target, self.min_sdk_version())?;
             cargo.args(self.cmd.args());
             if !cargo.status()?.success() {
                 return Err(NdkError::CmdFailed(cargo).into());
             }
         }
         Ok(())
     }
+
+    /// Returns `minSdkVersion` for use in compiler target selection:
+    /// <https://developer.android.com/ndk/guides/sdk-versions#minsdkversion>
+    ///
+    /// Has a lower bound of `23` to retain backwards compatibility with
+    /// the previous default.
+    fn min_sdk_version(&self) -> u32 {
+        self.manifest
+            .android_manifest
+            .sdk
+            .min_sdk_version
+            .unwrap_or(23)
+            .max(23)
+    }
 }