[Roll Forward] Block background custom toasts

Previous CL (ag/9699132) broke CTS ToastTest and was reverted (ag/9754500).
This was because the test was posting a background toast and verifying it
wasn't clickable. Test has been updated to use a foreground toast in ag/9754274
and I will also look into having it be executed in presubmit.(b/144810971).

-- Previous CL description (updated "Test:" lines)

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, 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: atest CtsToastTestCases CtsWindowManagerDeviceTestCases:ToastTest
      CtsWidgetTestCases:ToastTest CtsToastLegacyTestCases
Test: Working on CTS
Bug: 128611929

Change-Id: I25c6339c6adeb907878596332f57e2fa229bfda9

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