Linux and native language

Question

Is it true that Android is a version of Linux? I have read that it is. Linux is a re-written version of Unix, right? Unix was written in C, right? I assume that Linux was written mostly or entirely in C++.
 
The thing that is confusing to me is that most programming for Android is done in Java. It appears that even MonoDroid executes Java. This seems strange to me. It seems to me that it would be more efficient if the MonoDroid runtime were written in C++ instead of Java.