absl_log.h is supposed to be a mirror of log.h that only uses ABSL_
prefixed macros. This change adds ABSL_VLOG_IS_ON to absl_vlog_is_on.h
and uses it in the implementation of VLOG. VLOG_IS_ON is then only
exported though vlog_is_on.h.

PiperOrigin-RevId: 588435577
Change-Id: Ifa9193afa0b782194b64447f4fedfaf24b72c95a