Add system_server to readtracefs group

This allows system_server to search/read tracefs entries. It is needed
for attaching cpu timeinstate bpf programs to tracepoints.

Bug: 208892266
Bug: 209513178
Bug: 214061655
Test: libtimeinstate_test
Change-Id: I4139605eb7c5277887092b3d6a3fb26bf4f8f171
Merged-In: I4139605eb7c5277887092b3d6a3fb26bf4f8f171