Lots of mobile apps are written in Objective-C, and Objective-C is C. A superset of C to be exact.

Since it's a single hybrid language, it's trivial to remove slower features fro, performance-intensive parts.

See my UIKonf talk https://www.youtube.com/watch?v=kHG_zw7%205SjE&feature=youtu...

Or my book: https://www.amazon.com/gp/product/0321842847/ref=as_li_tl?ie...

Most Android apps use a handful of native code libraries which are bundled inside the apk. It's usual nowadays to only write the UI using Java and use C for the rest.

Linux is just the kernel for Android, the whole userspace is not written in C, and that is what matters in the end for performance and efficiency.