gril: Switch process id to radio user

Switching process id to radio user allows us to connect to rild without
the need to apply a patch for removing the id check in the Android
sources. After connecting to rild, we switch back to root user.

Based on Jolla code (https://github.com/nemomobile-packages/ofono,
commit 87f4764bb3d4af5c65d3fde875dbdc25978cba9d)