Cleanup references to "Day 4" (#603)

* Align outline with new spin-off course structure

With the new structure, the section on Android is a spin-off course
from the main 3-day course on Rust Fundamentals. The Bare-metal and
Concurrency days are spin-off courses in the same way.

* Explain new course structure

* Align Bare-Metal welcome page with other deep dives

* Merge Day 4 page into Course Structure page

* Remove Day 4 Welcome page

This aligns the Concurrency in Rust section with the Bare-Metal Rust
deep dive.

* Show subsections for Android deep dive

This aligns the Rust in Android section with the other deep dives.

* Clean up welcome page and README

We now cover async Rust and the course is no longer a four day course.

* Remove reference to the old Day 4

* Remove Day 4 references from exercises

src/exercises/android/morning.md

@@ -0,0 +1,15 @@
+# Exercises
+
+This is a group exercise: We will look at one of the projects you work with and
+try to integrate some Rust into it. Some suggestions:
+
+* Call your AIDL service with a client written in Rust.
+
+* Move a function from your project to Rust and call it.
+
+<details>
+
+No solution is provided here since this is open-ended: it relies on someone in
+the class having a piece of code which you can turn in to Rust on the fly.
+
+</details>