[nunit] build X.A.NUnitLite.dll with API level 19

[radekdoulik]

• current NUnitLite was built with latest API level. that was
  problematic, because it was referencing BaseBundle class, which is
  only available in API >= 21, see
  https://developer.android.com/reference/android/os/BaseBundle.html

• so instead of using latest API, we now build it with API 19. that's
  the lowes API level we can use, because
  src/Xamarin.Android.NUnitLite/Gui/Instrumentations/TestSuiteInstrumentation.cs
  is using global::Android.OS.Environment.DirectoryDocuments which is
  avalable in API level >= 19, see
  https://developer.android.com/reference/android/os/Environment.html#DIRECTORY_DOCUMENTS

• also the X.A.Build.Tasks project reference is now conditional,
  because it requires API level >= 24. it is OK, because we build it
  in leeroy-all rule with latest API level and default build uses
  latest API level too. the project reference itself is there to
  ensure the build ordering, so we can safely make it conditional

• it fixes bugs #53418 and #53884

[jonpryor]

Unit test execution failed on macOS+xbuild PR Build, and I'm not at all sure why.

    Task "CheckAdbTarget"
              [Output] IsValidTarget: True

There's an emulator...

            Tool /Users/builder/android-toolchain/sdk/platform-tools/adb execution started with arguments:   uninstall "Mono.Android_Tests" 
              [Output] Output:
                Error: Could not access the Package Manager.  Is the system running?

...but we can't access the package manager?

All following adb commands fail with the same message.

Re-reading, it gets weirder:

    Target AcquireAndroidTarget:
    Task "CheckAdbTarget"
            Using task CheckAdbTarget from Xamarin.Android.Tools.BootstrapTasks.CheckAdbTarget, Xamarin.Android.Tools.BootstrapTasks, Version=1.0.6304.13713, Culture=neutral, PublicKeyToken=null
            Task CheckAdbTarget
              AdbTarget: 
              SdkVersion: 
            Tool /Users/builder/android-toolchain/sdk/platform-tools/adb execution started with arguments:  shell getprop ro.build.version.sdk 
            Environment variables being passed to the tool:
            21
            Tool /Users/builder/android-toolchain/sdk/platform-tools/adb execution finished.
              [Output] AdbTarget: 
              [Output] IsValidTarget: True
    Done executing task "CheckAdbTarget"
    Done building target "AcquireAndroidTarget" in project "/Users/builder/jenkins/workspace/xamarin-android-pr-builder/xamarin-android/tests/RunApkTests.targets".

We never create an emulator. A device is already attached, as far as AcquireAndroidTarget is concerned, so it doesn't create one.

This is very fishy; I'm going to kick off a new build.

[jonpryor]

I don't think > needs escaping in XML. It can be, but I don't believe it's required, and >= is certainly easier to read...

[radekdoulik]

it was probably escaped by XS or emacs. will try to use >

[jonpryor]

build

[jonpryor]

The rebuild worked. I don't know why it failed last time.

[radekdoulik]

ok, removed the escaping