[Memory Panel] Fix canvasChartWidth has not been initialized error when changing the zoom duration

[elliette]

Fixes #8187

Fixes the "canvasChartWidth has not been initialized" error which can occur when setting the zoom duration in the Memory Panel.

---

Explanation

Setting the zoomDuration for a chart triggers a call to canvasChartWidth, which is only set after a chart has been painted:

In paint():

devtools/packages/devtools_app/lib/src/shared/charts/chart.dart

Lines 158 to 161 in 2f73978

	if (size != chartController.size) {
	chartController.size = size;
	chartController.computeChartArea();
	}

In computeChartArea():

devtools/packages/devtools_app/lib/src/shared/charts/chart_controller.dart

Lines 409 to 411 in 2f73978

	// Compute max number of ticks visible on X-Axis.
	visibleXAxisTicks = (width / tickWidth).truncate();
	canvasChartWidth = visibleXAxisTicks.toDouble() * tickWidth;

However, not all charts are painted by default. The Android chart is only conditionally painted:

devtools/packages/devtools_app/lib/src/screens/memory/panes/chart/widgets/chart_pane.dart

Lines 166 to 170 in 2f73978

	if (widget.chart.isAndroidChartVisible.value)
	SizedBox(
	height: defaultChartHeight,
	child: MemoryChart(widget.chart.android, sampleAdded),
	),

Therefore, the fix here is to only set the zoomDuration for the Android chart if it's visible.