Add support for Java 15

Conscrypt seems to not work with the latest Java15 release:

```
Caused by: javax.net.ssl.SSLPeerUnverifiedException: Could not find class: java.lang.ClassNotFoundException: com/sun/security/cert/internal/x509/X509V1CertImpl
at org.conscrypt.SSLUtils.toCertificateChain(SSLUtils.java:338)
... 31 common frames omitted
```

This is most likely caused by:

https://github.com/openjdk/jdk/commit/d243e40f7b000d3f46a63da66ce55bc790706657#diff-06dcd66603a23837e5639cdccee0d1ea

As this commit removed the class.

```
/Users/norman/Downloads/jdk-15.jdk/Contents/Home/bin/java -version
openjdk version "15-ea" 2020-09-15
OpenJDK Runtime Environment (build 15-ea+20-899)
OpenJDK 64-Bit Server VM (build 15-ea+20-899, mixed mode, sharing)
```

This showed up while adding support for running our testsuite in netty with Java15:

https://github.com/netty/netty/pull/10210