Move SqliteEngine.UseWinSqlite3 to Internal for now

[bricelam]

Given the issues mentioned in #249, the whole point of using winsqlite3.dll is kind of moot. We should remove it for RTM and look at better ways of enabling it in the future.

[natemcmaster]

But the API works on desktop for Windows 10, right?

[bricelam]

It "works", but .NET Native still requires sqlite3.dll to be present and loads it, so there's no point in using the in-the-box version.

[bricelam]

Re-marking for triage. As @natemcmaster pointed out, this would also cut the feature for non-.NET Native, Windows 10 users who are currently using it. (Since it also works outside of UWP.)

[divega]

@natemcmaster suggested in the PR that we could make the API internal for RTM, and I personally believe that may be an acceptable compromise as it means we can eventually remove it if we come up with something different.