Drop support for openjdk 1.7, reconsider current sdk versions

Is anyone still using openjdk 1.7 in the wild with Conscrypt?

Openjdk and Android builds have separate settings for source compatibility so we can do this with no Android impact and drop the openjdk 1.7 separate test suite and CI runs.

Separately, we might be able to update the source compatibility on Android to 1.8, which would be nice as the current Android toolchain should be able to desugar to 1.7 target compatibility. We'll still need to be careful with classes unavailable in the platform on earlier Android (such as `SniHostname`).

Currently we're building for Android unbundled with minSdkVersion=9 (Gingerbread) and targetSdkVersion=26 (Oreo) We
should definitely update target sdk to something newer and currently the oldest available emulator images in Android Studio seem to be sdk 15 (Ice Cream Sandwich) so maybe it's time to update our minSdkVersion.

Dashboard stats seem to have moved into the Studio "Create Project" wizard which doesn't offer minSdk versions lower than 16 and says that that level covers 99.8% of devices.