[Performance] Allow manual initialization of expensive operations to potentially decrease startup time

[kingces95]

To generate this profile you need to build a private nuget package with this branch and then build https://github.com/kingces95/droidperf.

Goal is to reduce startup time by making renderer registration explicit and allowing for feature opt out and to build in a profiler to allow adding regression tests.
NO BREAKING CHANGES -- All features are opt in: A new Init call + profiler logic lives behind a define.

To generate the nuget:

set NUGET_DIR=x:\nuget\4.4.1
.create-nuget.bat rdroid
X:\git\forms.nuspec\Xamarin.Forms.9.9.46.nupkg

To build the droidperf:

• Install VS Ent -- only that supports AOT
• Config = Release
• Build/Deploy XFFastUp.Android

Profiler API:
The profiler API is built in but hidden behind a PROFILE define. The define is set in this branch to ease development. Will remove before accepting PR. To inject sampling points simply put:

Profile.Push("Timing Partition A")
... // logic to time
Profile.PopPush("Next Partition")
... // more logic to time
Profile.Pop()
To report on the samples use the droidperf app linked above.

Explicit Registration:
This is an example of using explicit renderer registration and optional opt-out of features.

• Handlers is an array of ExportRendererAttribute
• Flags is a list of features to opt out of using
  Taken from: https://github.com/kingces95/droidperf/blob/23f8a333c08fd9f346ef3f4ef93ebfbbe68bc23a/XFMinUp/XFFastUp.Android/MainActivity.cs#L41

var activation = new ActivationOptions()
{
	Activity = this,
	Bundle = savedInstanceState,
	ResourceAssembly = Assembly.GetExecutingAssembly(),
	Handlers = Exports.Handlers,
	Flags = ActivationFlags.NoCss
};
Forms.Init(activation);
//Forms.Init(this, savedInstanceState, Assembly.GetExecutingAssembly());

[mattleibow]

Nice!

[KSemenenko]

any updates?

[hartez]

Looks good, just a couple of questions and a typo.

[kingces95]

Got the go ahead to check this in behind a PROFILE define. So I'll address the concerns, double check ensure everything is behind the PROFILE define, then get a final review, and merge it in... that's my plan.

[kingces95]

Eh, changed my plan. I'm not gonna hide behind a PROFILE.

Instead, I'll add Start. Without calling Start all the public entry points are noops and are small enough so they should be in-lined so that their cost will minimal and allow us to simply ship the profiler disabled in the product.

[kingces95]

Build

[samhouts]

I'm re-running the Fast Renderers tests. The other failing tests are not related to this PR.

[hartez]

Question about commented code.

[hartez]

Why are all these Profile calls commented out?