screencap: Fix parsing of display IDs

The underlying type of DisplayId is uint64_t, but screencap parsed the
IDs as signed 64-bit, invoking undefined behavior for out-of-range IDs.
In practice, out-of-range IDs were clamped, so SF failed to find them.

Fixes: 302580952
Test: screencap works for virtual display on Felix
Change-Id: I09a863d0c68dbb857b6f756b51159e5e3d853f5d