cargo-apk: Use ndk-gdb from the NDK root directory instead of expec…

…ting it in $PATH (#257)

Hardly anyone has the root directory of the NDK in their `$PATH` or
installed globally.  Thus, just like any other tool, invoke it from the
detected and selected NDK directly.

cargo-apk/src/apk.rs

@@ -241,7 +241,9 @@ impl<'a> ApkBuilder<'a> {
             jni_dir.join("Android.mk"),
             format!("APP_ABI=\"{}\"\nTARGET_OUT=\"\"\n", abi.android_abi()),
         )?;
-        Command::new("ndk-gdb").current_dir(target_dir).status()?;
+        Command::new(self.ndk.ndk().join("ndk-gdb"))
+            .current_dir(target_dir)
+            .status()?;
         Ok(())
     }