Make sure we cannot use packages targeting pre-O as WebView packages.

Using a WebView package targeting a release before O as WebView provider
on O will cause crashes (because of incompatibility issues), this CL
makes us interpret a package targeting a pre-O release as being invalid
on O.

Also remove the code that lets us fall back to loading the old
WebViewChromiumFactoryProvider (pre-O) class.

Bug: 34773740
Bug: 34180497
Test: Ensure WebView provider with targetSdkVersion="O" shows up in the
WebView Implementation Dev Setting, ensure targetSdkVersion=25 doesn't
show up.
Test: Add WVUS unit test to ensure a packages with targetSdkVersion < 26
are considered invalid, and > 26 are valid. Run WVUS unit tests.

Change-Id: I4d80d46019e2522bc3fc6068712d28eedb31fcce