Update Flutter samples about codemagic-sample-projects

[TahaTesser]

Also "Install Pods" is included in the "Android Workflow".
iOS commands should be removed from Android workflows.

codemagic-sample-projects/Firebase-App-Distribution/codemagic_firebase_app_distribution_yaml-master/codemagic.yaml

Lines 45 to 47 in d1e842b

	- name: Install pods
	script: |
	find . -name "Podfile" -execdir pod install \;

[TahaTesser]

Just leaving this feedback for anyone updating Flutter samples

• Update SDK constraints and migrate code samples to null-safety.

• Remove trailing a lot of spaces from .yaml files and script files.

• Update codemagic.yaml and build.gradle to match in the same example.
  For instance:
  Here If I follow codemgic.yaml file to create environmental variables with some, I would have to update build.gradle to match the same example. These two files should be using the same variable in the same example.

  codemagic-sample-projects/flutter/flutter-android-and-ios-yaml-demo-project/codemagic.yaml

  Lines 6 to 16 in 271c020

  	groups:
  	# Add the group environment variables in Codemagic UI (either in Application/Team variables) - https://docs.codemagic.io/variables/environment-variable-groups/
  	- keystore_credentials # <-- (Includes CM_KEYSTORE, CM_KEYSTORE_PASSWORD, CM_KEY_PASSWORD, CM_KEY_ALIAS)
  	# CM_KEYSTORE <-- Put your keystore file here
  	# CM_KEYSTORE_PASSWORD <-- Put your keystore password here
  	# CM_KEY_PASSWORD <-- Put your keystore alias password here
  	# CM_KEY_ALIAS <-- Put your keystore alias username here
  	- google_play # <-- (Includes GCLOUD_SERVICE_ACCOUNT_CREDENTIALS - Put your google-services.json here)
  	- other
  	vars:
  	FCI_KEYSTORE_PATH: /tmp/keystore.keystore

  
  

  codemagic-sample-projects/flutter/flutter-android-and-ios-yaml-demo-project/android/app/build.gradle

  Lines 52 to 61 in 271c020

  	if (System.getenv()["CI"]) { // CI=true is exported by Codemagic
  	storeFile file(System.getenv()["FCI_KEYSTORE_PATH"])
  	storePassword System.getenv()["FCI_KEYSTORE_PASSWORD"]
  	keyAlias System.getenv()["FCI_KEY_ALIAS"]
  	keyPassword System.getenv()["FCI_KEY_PASSWORD"]
  	} else {
  	storeFile file("/path/to/local/myreleasekey.keystore")
  	storePassword System.getenv()["KEYSTORE_PASSWORD"]
  	keyAlias System.getenv()["KEY_ALIAS_USERNAME"]
  	keyPassword System.getenv()["KEY_ALIAS_PASSWORD"]

[K-Codemagic]

Projects are updated.