init: Put init in group AID_READPROC

bootcharts currently only show root's processes, which isn't very
useful. To investigate and track boot duration issues, we need a way for
init to see all the pids. Add init to GID 3009 (aka AID_READPROC)

Ensure that init's children don't inherit this GID by always clearing
supplementary group IDs on fork.

Bug: 32506197
Test: Device boots and /proc/1/status says init is in gid 3009
Test: zygote starts and the Group: line in /proc/ZYGOTEPID/status is empty
Change-Id: Iba90717aaa591f1d6030a379a272aee003600c0a
Not-Tested: bootchart actually works. Speculative fix.