[msbuild] Always codesign app bundles if the codesign key is non-null

[jstedfast]

...even if it is a simulator build.

Turns out that starting with Xcode9, sim builds need to be codesigned
for App Groups entitlements to work properly. Interestingly, the
DetectSigningIdentity logic had a comment about needing to codesign
simulator builds for some entitlements to work already starting with
Xcode 8 but apparently the iOS targets did not respect this.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=59379

[jstedfast]

It should be noted that based on my tinkering around yesterday, it seems that Xcode9 always codesigns simulator builds and it's not based on what entitlements are enabled or anything like that.

Based on a comment in DetectSigningIdentity, it seems that it might be that way for Xcode8 as well but I have not verified that yet (will be doing shortly and adding a follow-up comment with my findings).

[jstedfast]

afaict, Xcode8 always seems to codesign sim builds as well.

[monojenkins]

Build success

[rolfbjarne]

This looks good, except that I think it should go into the xcode9 branch instead, otherwise it will take a while before customers get this fix.

[jstedfast]

@rolfbjarne ok, I'll submit a PR against xcode9. I wasn't sure which branch I should make it against before :)