Allow custom toasts only if app has resumed activity

Bug linked is a vulnerability where an app starts an external activity
that it wants the user to interact with along with repeatedly posting
custom toasts with the intent of redressing the UI of the activity just
launched.

The activity that is being started is translucent, so the check for
IMPORTANCE_FOREGROUND wasn't working because activity manager was still
considering the process foreground since it had a visible activity.

Thus, changing our logic to only enable custom toasts in case the app
has a resumed activity. This has the following implications in the
cases below:
* Translucent activity on top: Block toasts
* Multi-window: Allow toasts
* Bubble: Allows when bubble is expanded (it's a resumed activity)
* SAW: Probably block, but fine since app can already do what they want
* onCreate(), onStart(), onResume(): Allow toasts
* onPause(), onStop(), onDestroy(): Block toasts

Note that custom toasts are deprecated and we haven't specified what
exactly "foreground" or "background" meant in this context, so we have
some flexibility in the implementation.

Bug: 115385786
Test: From an app start a translucent activity from another package and
      then try to post a toast, verify the toast is blocked.
Test: atest ToastUITest android.widget.cts.ToastTest
      android.widget.cts29.ToastTest android.server.wm.ToastTest
Change-Id: Ia434332e066f1ef2cd01e150b087f8e5117f1e63