Fix SecurityException in Editor.onDrop

When a content URI is dropped onto EditText, it tries making sense
of the contents, and in the process it accesses the content provider.
If this content provider requires a permission grant, SecurityException
occurs.

This fix does two things:
1. Editor.onDrop now requests DropPermissions and releases is
when it is done. This required the introduction of a new hidden method
DropPermissions.takeTransient, because the existing method required
access to an Activity instance.

2. If the drag originator neglected to allow the permission grant,
DropPermission request would not help, so a try/catch block
is added to Editor.onDrop to avoid breaking the recipient app.

Bug: 26694948
Change-Id: I714429a507e62c83a150d91fbcdee791bced3ad3