Benefits of compiling C code with gcc's C++ front-end
I can only speculate, but considering how the Android system has grown in complexity, the scoping features of C++ (classes and namespaces) might make the code base more manageable.
EDIT
Even if the project doesn't currently make use of any C++ features, they may simply be planning ahead.
Apart from some minor differences (namely some parameter conventions most people avoid anyway), C source code compiles as C++ without modification. That being said, in some areas C++ syntax is stricter than C (C allows you to assign a void pointer to another pointer type without a cast; in C++, this is an error), and enforcing this strictness avoids problems down the road. *
*) (That's an overly simplistic view, see comment)
One further reason for the change may be that because most modern development favors C++ over C, a richer set of tools is available.
Speculating again, but at the birth of Android C may have been the only viable option for embedded device development, and now that restriction is no longer an issue.