[Xamarin.Android.Build.Tasks] Use @(AndroidDefineConstants)

[jonpryor]

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=56976

A "funny" thing happened in the migration from xbuild to msbuild:
F# compilation behavior changed.

This makes nearly no sense at all; it's the confluence of:

1. fsc.exe deosn't support specifying multiple symbols in a single
   fsc.exe --define. Specifically, unlike csc/mcs, the
   following command line does not define the symbols A and B;
   it is instead, effectively, ignored:

   fsc.exe "--define=A;B"
   

2. The <GetAndroidDefineConstants/> task generated a ;-separated
   string value.

3. xbuild differs from msbuild in ;-splitting behavior.

The <GetAndroidDefineConstants/> task is used to determine
additional conditional compilation symbols based on the API level. For
example, if an app sets $(TargetFrameworkVersion)=v2.3 (API-10),
then <GetAndroidDefineConstants/> will create the MSBuild property
$(AndroidDefineConstants) with the value:

__XAMARIN_ANDROID_v1_0__;__MOBILE__;__ANDROID__;__ANDROID_1__;__ANDROID_2__;__ANDROID_3__;__ANDROID_4__;__ANDROID_5__;__ANDROID_6__;__ANDROID_7__;__ANDROID_8__;__ANDROID_9__;__ANDROID_10__

This is provided as a courtesy to developers so that they can easily
conditionally include or exclude code based on the
`$(TargetFrameworkVersion) value.

When using xbuild, this works: the $(AndroidDefineConstants)
property is split on the ; to create an ITaskItem[], which is
provided to the <Fsc/> tasks' Fsc.DefineConstants property. This
in turn allows the <Fsc/> task to generate a sequence of --define
options, one per symbol:

fsc.exe --define=__XAMARIN_ANDROID_v1_0__ --define=__MOBILE__ --define=__ANDROID__ ...

Unfortunately, this behavior is an xbuild bug, in that msbuild
doesn't behave this way. Instead, msbuild doesn't split the
$(AndroidDefineConstants) property on ; to create an
ITaskItem[]; instead, it creates a single element which contains the
$(AndroidDefineConstants) value as-is. This causes the <Fsc/> task
to generate a command-line like:

fsc.exe "--define=__XAMARIN_ANDROID_v1_0__;__MOBILE__;__ANDROID__;..." ...

If this were csc, that would be fine, but it's not csc, and it's
not fine.

The result is that code that requires that e.g. __ANDROID__ be
defined no longer works as expected, becuase __ANDROID__ isn't
defined when building with msbuild.

The fix? If we need an ITaskItem[], we should create one, not create
a string and assume that msbuild will split on ;. Change the
GetAndroidDefineConstants.AndroidDefineConstants property from being
a string into an ITaskItem[], where each element of the array is a
separate symbol which should be defined. This allows the desired
values to be sent to the Fsc.DefineConstants property, allowing
compilation to work as desired.

[jonpryor]

Aside: while this adds a test, the test is effectively meaningless, because the test framework uses xbuild, not msbuild, and building under xbuild works as expected.

@dellis1972: Is there some way to force building with msbuild? Or should we just deal with that when we migrate the entire build system to actually use msbuild for unit test execution on the bots?

Additionally, it looks like the $USE_MSBUILD environment variable was intended to be used as a way to specify that msbuild should be used, but I don't see how that actually works; see Builder. GetXbuildPath(), which appears to be dead code.

[dellis1972]

@jonpryor there is an environment variable we can set to force the use of msbuild

USE_MSBUILD=1

[dellis1972]

formatting :P

[dellis1972]

can we remove this ? The output will be emitted automatically if it fails.