Don't block app-rendered text toast on apps with targetSdk < 30

We've decided to gate the text toast move on targetSdk due to an app
crash (b/148438364) but decided not to gate the background custom toast
block (in NMS) on targetSdk temporarily to gather dogfood and DP
feedback. This meant that app-rendered text toasts (which are only
possible for apps with targetSdk < 30) were being blocked in NMS since
they were being considered custom toasts.

This brings back the boolean inside Toast class that differentiates
between custom and text toasts for apps with targetSdk < 30, since both
are app-rendered in this case. This code won't be present on final
version and is temporary to gather feedback on dogfood and DP, since
almost no app currently targets SDK 30+. We'll gate both changes (the
system UI move and the bg custom toast block) on targetSdk later on.

Bug: 148761979
Bug: 148794147
Bug: 144754526
Test: Verify that, while on the background:
      App with       | Custom toast | Text toast
      targetSdk < 30 |      Blocked |    Allowed
      targetSdk 30+  |      Blocked |    Allowed
Test: atest android.widget.cts.ToastTest android.widget.cts29.ToastTest
            LegacyToastTest

Change-Id: Id529028bc463034cd2f64a6f2f97e71bc6ea7c36