logd: don't drop user/groups/capabilities/priority in drop_privs()

On Android, unlike POSIX, groups and capabilities are able to be set
per thread.  This is useless however, since threads are not a security
boundary.  This change drops the logic to set groups and capabilities
per thread and instead leaves all threads running with the initial
user and groups.

This does still drop some capabilities if they're unneeded due to
features being disabled.

This also moves the setpriority() call from code into the init script.

Test: logd runs with the expected user/groups and with the expected
      capabilities and priority without any errors

Change-Id: Ibb0e529ea1574a2b8ec391a2678504ca9fbe19be