Streamline testInstall_compatibleContext()

Conscrypt is making ClientSessionContext#getSession() private since
users shouldn't be calling it, which causes this test to fail to build.
While this test uses it, in practice the only non-Conscrypt code it's
testing is SSLSessionCache#install(), the rest of it just tests that
Conscrypt works properly.  We reduce the test to just a test of the
SSLSessionCache code, since Conscrypt's tests should suffice for testing
Conscrypt's use of the cache.

Test: adb shell am instrument -w -e class android.net.SSLSessionCacheTest com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Change-Id: Iea61c47eb2c002629dd2f50c81d5bc4f7221c654