liblog: reset pid and uid cache after a vfork()

Bionic getuid() and getpid() calls cache to reduce the need to
perform a syscall, and also reset their own cache after a vfork().
No more need for liblog to be performing this flawed cache operation.

Bug: 30085794
Change-Id: I70feed8bff0ddd919c2885a348ba67b14ddc0e0d