Release drag-and-drop URI permissions on gc if not tied to an activity

The implementation for drag-and-drop provides two ways of taking URI
permissions: a public API that ties permissions to an activity and
an internal API that grants "transient" permissions. The latter is
used in a single code path: dropping content into an editable
TextView (implemented in android.widget.Editor).

The transient code path was previously (Android R and below)
implemented using a try/finally block in Editor.onDrop. The change
I7ae38069fb741925211b42c7ff28784eb9722ce3 had updated that logic to
tie permissions to the app process lifecycle instead, to account for
the new OnReceiveContentListener which apps can use to implement
custom drop handling (and use a background thread to do the processing
of the content). The problem with having permissions tied to the app
process lifecycle is that the app process could potentially be
long-running.

This change updates the logic to tie transient permissions to the
DragAndDropPermissions object lifecycle (release whenever there are no
more reference to the object and it is garbage collected). This
follows the same approach as used for IME permission grants
(InputContentInfo.java and InputContentUriTokenHandler.java).

Bug: 181178998
Test: atest CtsWindowManagerDeviceTestCases:CrossAppDragAndDropTests
Test: Manually verified using development/samples/ReceiveContentDemo
Change-Id: I950d4572ee19af64da1ace1572fe12e17d09c609