fix(material/autocomplete): mark control as touched once panel is closed

[crisbeto]

Currently we mark the autocomplete control as touched on blur. The problem is that the blur event happens a split second before the panel is closed which can cause the error styling to show up and disappear quickly which looks glitchy. These changes change it so that the control is marked as touched once the panel is closed.

Also makes a couple of underscored properties private since they weren't used anywhere in the view.

Fixes #18313.

Caretaker note: Change is problematic, lots of tests depend on programmatic blur events to mark a control as touched.

[crisbeto]

Caretaker note: this changes the order of events slightly which has the potential to break tests. We should evaluate how to proceed based on the presubmit results.

[jelbourn]

SIGH

error TS2341: Property '_onChange' is private and only accessible within class 'MatAutocompleteTrigger'.

96       this.autoTrigger._onChange(this.autoTrigger.activeOption.value);

There are 110 failing targets. Half of them are build errors like this, and the other half are test failures due to the behavior change. We've got to get people using the harnesses.

[crisbeto]

I've put back the two functions so at least the compilation issues are resolved.