Block background custom toasts

To assess app compatibility problems early on, this CL blocks background
custom toasts in a non-secure way. We create a new method in the AIDL called
enqueueTextToast() that now contain the same parameters as
enqueueToast(), receiving the view created inside the app's process. But this
method will, in the future, contain a CharSequence instead of
ITransientNotification, for window creation inside the system.

We use PlatformCompat infrastructure to check if the change is enabled.

I learned with this change that if user blocks notifications for an app, it
won't be able to show background toasts, allowing me to re-use the foreground
check that was already in place.

This change will display a text toast (from the system) in case the app's
custom toast gets blocked, this is temporary to get feedback from
dogfood.

Test: With sample app targeting SDK 29, verify that:
      1. Posting text toast in bg works
      2. Posting custom toast in fg works
      3. Posting custom toast in bg does NOT work
Test: With sample app targeting SDK 28, verify that 1, 2 and 3 above work.
Test: Working on CTS
Bug: 128611929
Change-Id: Icecbbeff081fa5e7c4329bf3b383d6859520b517