Firestore: Improve Android tests to use the new env() helper function rather than creating an Env on the stack

[dconeybe]

Many of the Firestore integration tests for Android need an instance of jni::Env. The usual solution is create an instance as a local variable by simply declaring jni::Env env. This is a bit annoying because it adds clutter to the source code. Additionally, jni::Env instances are not allowed to be shared between threads, and having a local variable makes it easy to accidentally share instances with other threads.

To fix this, FirestoreAndroidIntegrationTest gains a new static member function named env() that returns a reference to an instance of jni::Env. By using a thread_local variable, it ensures that each thread gets its own instance. Then, (almost) all occurrences of jni::Env env have been removed from the tests, and where an instance was needed it just calls the env() function.

[github-actions]

Integration test with FLAKINESS (succeeded after retry)

Requested by @dconeybe on commit 99b39b0
Last updated: Fri Jun 23 12:11 PDT 2023
View integration test log & download artifacts

Failures	Configs
gma	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   FirebaseGmaTest.TestNativeAdLoadEmptyRequest
remote_config	[TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/2 android_device: android_target] (1 failed tests)   FirebaseRemoteConfigTest.TestAddOnConfigUpdateListener

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

[wu-hui]

LGTM.