Move is_system_uid utility from netd.c to netd.h

1. Move it to header file so that it can be reused by others.
2. Correct the return type from int to bool.
3. Replace __always_inline by inline to avoid -Werror,-Wunused-function.

Bug: 288340533
Test: build
Change-Id: I9062686d9c2f98c2d24e4673f82b1732b180ffc4