Fix VirtualInputDevice build for musl libc

The struct input_event timespec types come from the kernel headers,
which can vary in their exact definition based on which libc they
came from.  Use decltype to cast to the exact definition of the
fields.

Bug: 271946580
Test: m USE_HOST_MUSL=true host-native
Change-Id: Iddcc6ea7c1e5d70edf8c159ddaff585eb38ef7fa