security: Stabilize AdvancedTlsX509KeyManager.

[erm-g]

This PR is a part of 'Stabilize Advanced TLS' effort.
Clean up, improve javadoc, de-experimentalize of AdvancedTlsX509KeyManager, add a unit test (e2e already exists).

[matthewstevenson88]

Thanks for the PR @erm-g! A couple nits, and two other points:

1. This PR only de-experimentalizes the key manager. Should we amend the PR title accordingly?
2. Please address the failing tests.

[matthewstevenson88]

Please address the failing tests. :)

[erm-g]

Please address the failing tests. :)

Fixed (few styling things)

[matthewstevenson88]

We seem to be testing several distinct behaviors in the same unit test - can we break these up into separate smaller-scoped tests so that each unit test is testing an independent behavior?

Similarly above in the credentialSetting test.

[erm-g]

In this test we're just checking that we correctly put checkNotNull checks across the public api. So I grouped that under single unit test since I saw a similar pattern at MatcherTest -

grpc-java/xds/src/test/java/io/grpc/xds/internal/MatcherTest.java

Line 64 in 2bc4306

public void stringMatcher() {

credentialSetting is different - we check a sequence of changing (serverCert->clientCert->serverCert) so splitting it is possible, but will require a lot of boilerplate code (smth like wordy BeforeMethod before each test)

[ejona86]

Let's swap to FakeClock, but otherwise looks good.

[ejona86]

@matthewstevenson88, do you want us to wait for your approval before this goes in?

[matthewstevenson88]

Thanks @ejona86. LGTM, and ok to merge once the FakeClock change is done.