Auto merge of #14530 - mmatyas:android_injectedglue, r=larsbergstrom

Use `android-rs-injected-glue` on Android

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Due to changes in `android-rs-glue`, currently the Android build fails to start. As discussed [here](https://github.com/servo/servo/issues/13154), other than the `android-glue`, we now also have to inject some additional code too. This task is usually done by a helper program of `android-glue`, `cargo-apk`, but since Servo's build system is somewhat more complex than a regular small application, at the moment it'd be more clean to move the required internal `injected-glue` lib to a new repository, and add it as a dependency.

This patch adds a new dependency, `android-rs-injected-glue`, and integrates it into Servo. After applying this commit and #14528, Servo can be launched on Android. (It doesn't *work* yet as expected, though.)

Part of #13154.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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ports/servo/Cargo.toml

@@ -54,3 +54,4 @@ sig = "0.1"
 [target.'cfg(target_os = "android")'.dependencies]
 libc = "0.2"
 android_glue = "0.2"
+android_injected_glue = {git = "https://github.com/mmatyas/android-rs-injected-glue"}

ports/servo/main.rs

@@ -20,6 +20,8 @@
 #[cfg(target_os = "android")]
 #[macro_use]
 extern crate android_glue;
+#[cfg(target_os = "android")]
+extern crate android_injected_glue;
 extern crate backtrace;
 // The window backed by glutin
 extern crate glutin_app as app;
@@ -247,18 +249,19 @@ fn args() -> Vec<String> {
 }
 
 
-// This extern definition ensures that the linker will not discard
-// the static native lib bits, which are brought in from the NDK libraries
-// we link in from build.rs.
 #[cfg(target_os = "android")]
-extern {
-    fn app_dummy() -> libc::c_void;
+#[no_mangle]
+#[inline(never)]
+#[allow(non_snake_case)]
+pub extern "C" fn android_main(app: *mut ()) {
+    android_injected_glue::android_main2(app as *mut _, move |_, _| { main() });
 }
 
 
 #[cfg(target_os = "android")]
 mod android {
     extern crate android_glue;
+    extern crate android_injected_glue;
     extern crate libc;
 
     use self::libc::c_int;
@@ -272,7 +275,7 @@ mod android {
         redirect_output(STDERR_FILENO);
         redirect_output(STDOUT_FILENO);
 
-        unsafe { super::app_dummy(); }
+        unsafe { android_injected_glue::ffi::app_dummy() };
     }
 
     struct FilePtr(*mut self::libc::FILE);