Remove obsoleted tests and platformDir from Paparazzi config

[jrodbx]

Historically, Environment.platformDir referenced build.prop and the framework resources bundled under the corresponding $ANDROID_HOME/platforms/$VERSION/ directory.

Now that we've migrated to Google's layoutlib published artifacts, build.prop is provided by layoutlib-runtime and the framework resources by layoutlib-resources, removing the need for Environment.platformDir.

Consequently, four plugin tests are obsoleted and removed:

• a check to see if a missing $ANDROID_HOME/platforms/$VERSION/ folder is detected (by @luis-cortes)
• a check supporting upcoming sandboxed versions of the Android platform (by @alexvanyo)
• two checks determining whether compile and target SDKs have been set differently or left to the default (same) by the developer.

This last bullet point is important. In the past, if the IDE release of LayoutLib did not yet support a given Android SDK level, Paparazzi provided a limited escape hatch to allow the developer to downgrade the platform SDK used by Paparazzi like so:

Paparazzi(
  environment = detectEnvironment().run {
    copy(compileSdkVersion = 33, platformDir = platformDir.replace("34", "33"))
  },
)

Here's a recent example of this: https://github.com/cashapp/paparazzi/issues/1288#issuecomment-1939659871

With the move to Google's Layoutlib, this facility is no longer available. Still, it's likely (hopeful?) that Google will publish LayoutLib versions often enough (i.e., faster than the IDE release cycle) to allow quicker adoption of newer SDK levels in Paparazzi.