Disable -Wnullability-completeness on iOS and macOS.

[jonsimantov]

Description

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Disable -Wnullability-completeness on iOS and macOS, which is making the Obj-C++ build too verbose.

Also, during this PR's tests, the cached NDK version became corrupted. Added logic to the workflows to remove a bad cached NDK download, and allow the prereqs script to download the NDK normally.

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Testing

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Integration tests.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

❌  Integration test FAILED

Requested by @jonsimantov on commit 04d0fa4
Last updated: Mon Dec 6 15:25 PST 2021
View integration test log & download artifacts

Failures	Configs
database	[TEST] [FLAKINESS] [Android] [windows] [android_target] (1 failed tests)   FirebaseDatabaseTest.TestInfoConnected
firestore	[TEST] [ERROR] [Android] [ubuntu] [android_target] [TEST] [FLAKINESS] [Android] [macos] [android_target] (1 failed tests)   CRASH/TIMEOUT

Add flaky tests to go/fpl-cpp-flake-tracker