[Mono.Android-Tests] Fix stat(1) arguments on Linux (#801)

macOS uses *BSD command-line tools which often take different parameters
than the Linux ones. This is the case with **stat**(1) which not only expects
different command line parameters but also has different meaning of various
format placeholders (in this case `%z` means "file size in bytes" on macOS
but "file time stamp timezone in human readable format").

This fixes **stat**(1) invocation on Linux.

src/Mono.Android/Test/Mono.Android-Tests.projitems

@@ -16,7 +16,8 @@
   </ItemGroup>
   <Target Name="_RecordApkSizes" AfterTargets="DeployUnitTestApks">
     <Delete Files="$(MSBuildThisFileDirectory)..\..\..\TestResult-Mono.Android_Tests-values.csv" Condition=" '$(Configuration)' == 'Debug' " />
-    <Exec Command="stat -f &quot;stat: %z %N&quot; &quot;$(_MonoAndroidTestApkFile)&quot; > $(_MonoAndroidTestApkSizesInput)" />
+    <Exec Condition=" '$(HostOS)' == 'Darwin' " Command="stat -f &quot;stat: %z %N&quot; &quot;$(_MonoAndroidTestApkFile)&quot; > $(_MonoAndroidTestApkSizesInput)" />
+    <Exec Condition=" '$(HostOS)' == 'Linux' " Command="stat -c &quot;stat: %s %N&quot; &quot;$(_MonoAndroidTestApkFile)&quot; > $(_MonoAndroidTestApkSizesInput)" />
     <Exec Command="unzip -l &quot;$(_MonoAndroidTestApkFile)&quot; >> $(_MonoAndroidTestApkSizesInput)" />
     <ProcessPlotInput InputFilename="$(_MonoAndroidTestApkSizesInput)" ApplicationPackageName="$(_MonoAndroidTestPackage)" ResultsFilename="$(MSBuildThisFileDirectory)..\..\..\TestResult-Mono.Android_Tests.xml" DefinitionsFilename="$(MSBuildThisFileDirectory)apk-sizes-definitions-$(Configuration)$(_AotName).txt" AddResults="true" />
   </Target>