Remove sendCloseSystemDialogs() from PhoneFallbackEventHandler

Today's flow of events is like this: key goes to focused app
(ViewRootImpl::processKeyEvent), the view hierarchy does not handle the
event, so ViewRootImpl uses FallbackEventHandler to invoke the fallback
action for this key.

The problem is that for many keys the app itself is closing system
dialogs before taking the appropriate action (often launching an
activity, eg. dialer for KEYCODE_CALL), but this is now prohibited in S
due to abuse of said action.

The long-term plan is to return to InputDispatcher the fact that the key
wasn't handled by the app and have ID call out to the policy, which will
launch the appropriate action and adjust the UI as it sees fit (close
system dialogs).

However, we need to prevent apps from crashing because of this, hence
this change for S still. The unfortunate effect is that system dialogs
won't be hidden in these cases, but this livable with until we properly
implement the infrastructure.

Bug: 199173862
Test: Simulate code-paths with affected keys and make sure apps don't
      crash:
      1. adb shell input keyevent --longpress KEYCODE_CALL
      2. adb shell input keyevent --longpress KEYCODE_CAMERA
Change-Id: I44ad41ac1eac9acc8320298ceb4c1b21bde8af5d