logtail: add Config.Disabled to suppress the startup banner

NewLogger unconditionally writes a "logtail started" banner before
it returns, which callers that later call Logger.SetEnabled(false)
have no way to suppress: the banner is already buffered for upload
by the time the caller gets the logger back.

Add Config.Disabled so callers that know up front they want the
logger to start disabled (e.g. Android's remote-logging opt-out)
can seed the state before NewLogger's internal Write. The process-
wide Disable kill switch still takes precedence; SetEnabled can
still flip the state at runtime.

Updates #13174
Updates tailscale/tailscale-android#695

Change-Id: Icc4fa88c198447cf0faa707264dac84e359fe52c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/logtail.go

@@ -132,6 +132,7 @@ func NewLogger(cfg Config, logf tslogger.Logf) *Logger {
 	}
 	logger.SetSockstatsLabel(sockstats.LabelLogtailLogger)
 	logger.compressLogs = cfg.CompressLogs
+	logger.disabled.Store(cfg.Disabled)
 
 	ctx, cancel := context.WithCancel(context.Background())
 	logger.uploadCancel = cancel