update gradle-plugin@2.3.3, gradle@3.5, gradle-download-task@3.4.0

[dulmandakh]

Now we use build tools version 26, and makes it possible to use android gradle plugin 2.3.3 (requires build tools 25), gradle 3.x for improved build performance and developer experience. Also it will help targeting SDK 26 easier.

Android gradle plugin to 2.3.3 requires gradle version 3.3 or above, and gradle download task version 3.2.0 and above supports gradle version 3.3. Therefore, it's better to use latest gradle 3.x or 3.5, and gradle download task 3.4.0.

Test Plan

React Native and apps will build much faster and run as before.

[pull-bot]

	Warnings
⚠️	📋 Release Notes - This PR appears to be missing Release Notes.

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[gengjiawen]

Why not just 3.1.2 and gradle 4 ?

[dulmandakh]

it has no breaking changes, but performance improvements. if we move to gradle plugin 3 or above, we need to deal with aapt2 and other breaking changes. as for now, aapt2 breaks compilation, and need to disable it explicitly.

[gengjiawen]

Then for me it's not neccessary to upgrade a middle version. And I thinks it's total okay to disable aapt2 since google not put much attension to it.

[dulmandakh]

You're free to choose your gradle and gradle plugin version for your project, it'll override react native. I use build tools 27 and latest gradle for my own.

But for react native, we need smooth transition because many people depend on it

[dulmandakh]

new PR #19697