Fix alarms with negative or very large wakup times.

When the wakeup time is negative, the kernel /dev/alarm driver
never triggers the alarm.  This can cause alarms to back up in the
priority queue since an alarm at the head with a negative wakup time
will never be triggered.  Now we use 0 as the wakup time which causes
an immediate triggering.

When the wakeup time is very large, it is possible for a numeric
overflow to occur when converting the timestamp from milliseconds
since epoch to nanoseconds.  This has been fixed by avoiding the
intermediate conversion in the JNI call so that overflow cannot
occur.

Bug: b/2558820
Change-Id: I4f5b4646a04090cc749a9fc5d3982a68402954ef