Remove default for `stripped` option, don't strip by default on android

[gmackall]

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This is round 3, after https://github.com/flutter/engine/pull/50443 and https://github.com/flutter/engine/pull/51394. I think this is finally the right place to make the change, if this gn actually works.

Always respect the stripped argument if it is passed, but otherwise default to not stripping on android, and stripping on other platforms.

Still need to do some testing before landing, but making the PR so that this stays on my radar

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[gmackall]

Picking this back up.

Need to figure out why AGP doesn't strip the resultant appbundle properly when the NDK version isn't downloaded.

[jmagman]

@gmackall post-monorepo can you recreate this in the framework and close this engine PR (especially since you landed the ndk download)?

[gmackall]

https://github.com/flutter/flutter/pull/161546