Update CI config for Flutter 2 (flutter#3674)

Includes cleanup to simplify our setup. Major changes:
- Eliminate the NNBD plugin filtering for stable.
- Remove the temporarily-added beta branch testing.
- Enable Linux, macOS, and web on stable (Windows is LUCI-based)
- Combine the two different macOS matrix configurations now that they
  are the same.
- Combine the two different Linux matrix configurations by using a single
  Dockerfile (which now also includes clang-format)
  - The web integration smoke test temporarily still uses the old Dockerfile,
    now renamed, because the driver installer script doesn't support
    Chrome 89 yet.
- Move most of the Linux tasks to lower-CPU machines to allow more
  tasks to run in parallel without hitting the community limit.
- Reorder the tasks slightly and give them comments to identify
  platform groupings
- Enabled web "build all plugins together" and "build all examples"
  tests

.ci/Dockerfile

@@ -6,7 +6,7 @@ RUN sudo apt-get install -y --no-install-recommends gnupg
 
 # Add repo for gcloud sdk and install it
 RUN echo "deb [signed-by=/usr/share/keyrings/cloud.google.gpg] https://packages.cloud.google.com/apt cloud-sdk main" | \
-    sudo tee -a /etc/apt/sources.list.d/google-cloud-sdk.list 
+    sudo tee -a /etc/apt/sources.list.d/google-cloud-sdk.list
 
 RUN curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | \
     sudo apt-key --keyring /usr/share/keyrings/cloud.google.gpg add -
@@ -23,7 +23,21 @@ RUN yes | sdkmanager \
 
 RUN yes | sdkmanager --licenses
 
+# Install formatter.
+RUN sudo apt-get install -y clang-format
+
+# Install xvfb to allow running headless
+RUN sudo apt-get install -y xvfb libegl1-mesa
+# Install Linux desktop build tool requirements.
+RUN sudo apt-get install -y clang cmake ninja-build file pkg-config
+# Install necessary libraries.
+RUN sudo apt-get install -y libgtk-3-dev libblkid-dev liblzma-dev
+
 # Add repo for Google Chrome and install it
 RUN wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo apt-key add -
 RUN echo 'deb [arch=amd64] http://dl.google.com/linux/chrome/deb/ stable main' | sudo tee /etc/apt/sources.list.d/google-chrome.list
 RUN sudo apt-get update && sudo apt-get install -y --no-install-recommends google-chrome-stable
+
+# Make Chrome the default so http: has a handler for url_launcher tests.
+RUN sudo apt-get install -y xdg-utils
+RUN xdg-settings set default-web-browser google-chrome.desktop

.ci/Dockerfile-LinuxDesktop → .ci/Dockerfile-LegacyChrome

@@ -6,7 +6,7 @@ RUN sudo apt-get install -y --no-install-recommends gnupg
 
 # Add repo for gcloud sdk and install it
 RUN echo "deb [signed-by=/usr/share/keyrings/cloud.google.gpg] https://packages.cloud.google.com/apt cloud-sdk main" | \
-    sudo tee -a /etc/apt/sources.list.d/google-cloud-sdk.list
+    sudo tee -a /etc/apt/sources.list.d/google-cloud-sdk.list 
 
 RUN curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | \
     sudo apt-key --keyring /usr/share/keyrings/cloud.google.gpg add -
@@ -15,17 +15,15 @@ RUN sudo apt-get update && sudo apt-get install -y google-cloud-sdk && \
     gcloud config set core/disable_usage_reporting true && \
     gcloud config set component_manager/disable_update_check true
 
-# Install xvfb to allow running headless
-RUN sudo apt-get install -y xvfb libegl1-mesa
-# Install Linux desktop build tool requirements.
-RUN sudo apt-get install -y clang cmake ninja-build file pkg-config
-# Install necessary libraries.
-RUN sudo apt-get install -y libgtk-3-dev
+RUN yes | sdkmanager \
+    "platforms;android-27" \
+    "build-tools;27.0.3" \
+    "extras;google;m2repository" \
+    "extras;android;m2repository"
 
-# Add repo for Google Chrome and install it, for url_launcher tests.
+RUN yes | sdkmanager --licenses
+
+# Add repo for Google Chrome and install it
 RUN wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo apt-key add -
 RUN echo 'deb [arch=amd64] http://dl.google.com/linux/chrome/deb/ stable main' | sudo tee /etc/apt/sources.list.d/google-chrome.list
 RUN sudo apt-get update && sudo apt-get install -y --no-install-recommends google-chrome-stable
-# Make it the default so http: has a handler.
-RUN sudo apt-get install -y xdg-utils
-RUN xdg-settings set default-web-browser google-chrome.desktop