Add integration_test to flavor test project

[jmagman]

In anticipation for #89045, add integration_test to the flavors app.
Since it is the first plugin, handle the flavor build configurations in the Podfile.

Stop adding include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.Debug.xcconfig" to the xcconfig if there's already a Pods include path in the file. Pods-Runner.Debug.xcconfig will not be correct when the target name is not Runner or when the build configuration (flavors) is not Debug.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[flutter-dashboard]

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[jmagman]

+@blasten for the Android project change

[blasten]

LGTM

[cyanglaz]

LGTM