fix(SystemBars): use native safe area insets on Android

[theproducer]

Description

This PR implements native safe area inset handling as demonstrated in https://github.com/capacitor-community/safe-area

(CSS Safe Area vars are still intact for OS upstream)

Change Type

• Fix
• Feature
• Refactor
• Breaking Change
• Documentation

Rationale / Problems Fixed

closes #8343
closes #8329
closes #8287

Tests or Reproductions

Screenshots / Media

Platforms Affected

• Android
• iOS
• Web

Notes / Comments

[OS-pedrogustavobilro]

A few comments on stuff I noticed while looking at the existing community safe-areas plugin.

[OS-pedrogustavobilro]

Tested on several apps shared across different GitHub issues, as well as a few internal apps, all seems to be working well. Very nice!

[tafelnl]

Awesome! Thank you so much for working on this and merging it! 🚀

A few points I'd like to discuss:

• what is the main reason for using DOMContentLoaded over just using onPageCommitVisible?
  ^{see also: feat(android): Improving SystemBars inset handling #8268 (comment)}
  • Does this approach also work when navigating to content inside the webview that hasn't Capacitor loaded in?
• If you prefer using DOMContentLoaded and making JS responsible for this, couldn't we then skip the overhead of using evaluateJavascript and just let the native code know the value directly
  ^{see also: feat(android): Improving SystemBars inset handling #8268 (comment)}
• The initial value of hasViewportCover is set to false. In my testing this could lead to slightly jumping of content when the value switches from false to true. It might be good to set it to true (if the majority of users has that as the default). Or even better: allow to configure this initial value. For the community plugin I chose initialViewportFitCover
• I think it's wise to remove all of the setOnApplyWindowInsetsListener related logic inside the @capacitor/keyboard plugin.
  ^{see also: fix(android): restore content height when keyboard hides via insets l… capacitor-keyboard#57 and fix(android): restore content height when keyboard hides via insets l… capacitor-keyboard#57 (comment)}
• We might want to update the documentation. Stating something along the lines of:

  "Capacitor v8 takes care of patching the safe area related bugs in Chromium out of the box. Using viewport-fit="cover" should work as expected. Alternatively you could use CSS variable injection if you don't want to rely on the natively reported inset envs"

Thanks again for your work on this. Cheers!

[optimix]

I tested today's version of SystemBars.java and noticed that the webView is now always becoming smaller when the keyboard is being shown (compared to the version 8.2.0).
Is there any setting to display the keyboard on top of the webView and leave the webView size unchanged ?
I thought that setting android:windowSoftInputMode="adjustNothing" would help achieving this but this is not the case.

[optimix]

I had an issue with the latest code: the UI was jumping at startup because shouldPassthroughInsets is false at the beginning and then set to true once the onDOMReady() and then runOnUiThread() finish setting hasViewportCover.
I did a small change on my end to bypass this: 745560c

In order to programatically control the behaviour of resizing the web view or not when the keyboard changes, here is an example with a new setResizeWebViewMode() function allowing to control this:
0b4283d