Enable building of SampleApps solution in CI loop

[jonthysell]

Right now SampleApps is disabled because it runs out of space on the build machines. Mitigations:

• Build a smaller, common pch to be shared by our projects under vnext and our projects under packages
• Fix out nuget restore story to make sure we aren't duplicating downloads into different paths, and therby confusing msbuild into building things multiple times.

[chrisglein]

@jonthysell this should be unblocked

[jonthysell]

Manager: Why did you move this to M4?
Me: Build's haunted.
Manager: Excuse me?
Me (grabbing gun): Build's haunted.

[licanhua]

Disk should not be a problem anymore.

But it has other issue that some of them requires C# 7.2 or greater:
##[error]vnext\Microsoft.ReactNative.SharedManaged\JSValue.cs(100,36): Error CS8107: Feature 'readonly references' is not available in C# 7.0. Please use language version 7.2 or greater.
##[error]vnext\Microsoft.ReactNative.SharedManaged\JSValueReader.cs(330,9): Error CS8107: Feature 'default literal' is not available in C# 7.0. Please use language version 7.1 or greater.
##[error]vnext\Microsoft.ReactNative.SharedManaged\OneOf.cs(24,35): Error CS8107: Feature 'default literal' is not available in C# 7.0. Please use language version 7.1 or greater.
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