iris: add the android-app cdylib crate (I2, part 3)

iris/android-app is the concrete app RUST.md's I2 is judged against:
JNI_OnLoad, a Client implementing AndroidAppState, and new_view_peer
wrapping iris::android::new_peer's generic function in the plain
function pointer register_view_class needs. Its UI is tabs-ui::build,
unchanged from the winit example.

Deliberately excluded from the iris workspace (iris/Cargo.toml's new
`exclude`): android-view needs the NDK sysroot to link, so folding this
crate in would break `cargo build --workspace --all-targets` on the
host. It resolves as its own single-crate workspace instead, built
with `cd iris/android-app && cargo ndk -t x86_64 -P 26 build`.

Verified: cross-compiles and clippys clean for x86_64-linux-android
API 26; the host iris workspace (build/clippy/fmt/19 tests) is
unaffected. Not yet built: the Gradle shell (IrisView.java,
MainActivity, AndroidManifest) to actually install and run this on the
emulator -- next in RUST.md's I2.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

.gitignore

@@ -26,3 +26,4 @@ sessions/
 
 # iris, the in-house UI library, is vendored at iris/ and built by cargo.
 iris/target/
+iris/android-app/target/