When the $(DesignTimeBuild) flag changes, build.props is invalidated

[jonathanpeppers]

Steps to Reproduce

1. Build
2. Cause a Design-Time Build to occur
3. Build again
4. More things build than should

Context: #1933
Context: #1943

One of the issues I noticed while debugging the issue with #1933 is
that $(IntermediateOutputPath)build.props gets invalidated if
$(DesignTimeBuild) changes. build.props triggers alot of targets
to build completely again, so this is pretty bad for our build times
in an IDE...

Design-Time Builds can run quite frequently in VS Windows, and we
don't want to rebuild a bunch of things unnecessarily when the user
switches back to a regular build.

Expected Behavior

Design-time builds should not invalidate obj\Debug\build.props, and therefore:

Design-time builds should not trigger any of the following targets if there are no changes:

• _LinkAssembliesNoShrink
• _UpdateAndroidResgen

Actual Behavior

Design-time builds cause a future build (even with no changes) to be slower.

Version Information

Happens on master/d15-8.

VS bug #660393