Openjdk 7 CI broken

e.g. https://travis-ci.org/github/google/conscrypt/builds/752268589
```
org.conscrypt.ConscryptJava7Suite > org.conscrypt.javax.net.ssl.SNIHostNameTest.test_byteArray_Constructor FAILED
java.lang.NoSuchMethodError: javax.net.ssl.SNIHostName.([B)V
at org.conscrypt.javax.net.ssl.SNIHostNameTest.test_byteArray_Constructor(SNIHostNameTest.java:44)

org.conscrypt.ConscryptJava7Suite > org.conscrypt.javax.net.ssl.SSLParametersTest.test_SSLParameters_setServerNames_duplicatedNameThrows FAILED
java.lang.UnsupportedOperationException: Stub!
at javax.net.ssl.SNIServerName.(SNIServerName.java:24)
at javax.net.ssl.SNIHostName.(SNIHostName.java:23)
at org.conscrypt.javax.net.ssl.SSLParametersTest.test_SSLParameters_setServerNames_duplicatedNameThrows(SSLParametersTest.java:138)

org.conscrypt.ConscryptJava7Suite > org.conscrypt.javax.net.ssl.SSLParametersTest.test_SSLParameters_setServerNames_setNull_getNull FAILED
java.lang.UnsupportedOperationException: Stub!
at javax.net.ssl.SNIServerName.(SNIServerName.java:24)
at javax.net.ssl.SNIHostName.(SNIHostName.java:23)
at org.conscrypt.javax.net.ssl.SSLParametersTest.test_SSLParameters_setServerNames_setNull_getNull(SSLParametersTest.java:152)
```

Nothing has changed about these tests recently and they'e protected by `TestUtils.assumeSNIHostnameAvailable()` which previously prevented them from running on openjdk 7 (which tests whether the class is available). So I suspect a gradle change has made the Android stub classes available on openjdk 7 when previously they weren't.