Revise session handling about conscrypt HOT 5 CLOSED

Java does provide an SSLSessionContext.getSession API which gets in the way of this, but since the returned SSLSessions aren't useful for anything, always returning null seems a sound choice. At a glance, it appears Netty does this and always makes SSLSocket.getSession and SSLSocket.getHandshakeSession return child objects of the socket as described here.

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Comments on this unsubmitted CL are relevant.

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We should import some of the android tests to verify these changes.

https://android.googlesource.com/platform/libcore/+/master/luni/src/test/java/libcore/

then it's stuff like javax/net/ssl/SSLSocketTest.java

Subdirectories for many of the tests:
https://android.googlesource.com/platform/libcore/+/master/luni/src/test/java/libcore/java/security/
https://android.googlesource.com/platform/libcore/+/master/luni/src/test/java/libcore/javax/net/ssl/
https://android.googlesource.com/platform/libcore/+/master/luni/src/test/java/libcore/javax/crypto/

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FYI, Kenny's work on this was captured in these 2 CLs (the latter is to be applied on the former):

https://android-review.googlesource.com/#/c/309613/
https://android-review.googlesource.com/#/c/312011/

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FYI, I've merged the two CLs into a single commit on GitHub:

nmittler@6362f73

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