[Impeller] Update docs to describe Impeller is enabled by default on Android

[amir14a]

Impeller is no longer in preview on Android and from flutter 3.27 it enabled by default on Android.

This PR is updating docs that fix this section and describe that Impeller is enabled by default now and describe how to disable it on android.

Its contents is from Impeller page on flutter.dev website: https://docs.flutter.dev/perf/impeller

[google-cla]

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[jtmcdole]

Monorepo Migration Completed

TL;DR: Please migrate your PR to flutter/flutter

The flutter/engine repository has been migrated to the flutter/flutter repository. This PR will no longer be landed here and must be migrated. To migrate your PR to the flutter repository, please follow these steps:

1. Create a patch for this PR:

   • Generate a patch set that represents the changes in this PR. The exact method will vary depending on your PR's history. If your PR includes merges, it is highly recommended that you rebase it onto main first.

   git format-patch $START_COMMIT..$END_COMMIT --stdout > combined_patch.patch

2. Update the patch for the new engine location:

   • The engine source code now resides in the engine/ subdirectory within the flutter/flutter repository. You'll need to update the file paths in your patch accordingly.

   # Insert the `engine/` prefix to all paths. Note that this usage works on macOS and
   # linux versions of sed.
   sed -i.bak \
   -e 's|^\(diff --git a/\)\(.*\) b/\(.*\)|\1engine/\2 b/engine/src/flutter/\3|' \
   -e 's|^\(--- a/\)\(.*\)|\1engine/src/flutter/\2|' \
   -e 's|^\(\+\+\+ b/\)\(.*\)|\1engine/src/flutter/\2|' \
   combined_patch.patch

3. Checkout and set up your Flutter development environment:

   • Follow the instructions in Setting up the Framework development environment to check out the flutter/flutter repository.
   • Remember to rename the origin remote to upstream (e.g., git remote rename origin upstream).

4. Set up the engine development environment within the monorepo:

   • Follow the updated instructions in Setting up the engine environment, paying close attention to the changes in gclient setup and location.

5. Apply the patch to a new branch:

   • Create a new branch in your flutter/flutter repository.
   • Apply the updated patch to this branch:

   git apply combined_patch.patch

6. Open a new PR:

   • Open a new PR in the flutter/flutter repository with your changes.
   • Reference this original PR in the new PR's description using flutter/engine#<PR_NUMBER>.

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