Fix for a deadlock in JniResultCallback

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

JniResultCallback was using two separate locks, with no guarantee on which will be grabbed first. This could cause a deadlock, leading to ANRs. Fix this by just using a single lock.

Addresses issues like: firebase/quickstart-unity#1286

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Building locally, plus GHA 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 with FLAKINESS (succeeded after retry)

Requested by @a-maurice on commit 8f8718b
Last updated: Wed May 24 16:02 PDT 2023
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/2 android_device: emulator_ftl_target] (1 failed tests)   SmokeTest.TestObservesNewDocument
gma	[TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT

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