Do not convert time to float

The conversion of time to float causes weird behaviour. When the time is
received in palm rejection code, the time deltas do not match the
original values seen in Android.

Rather than debug the float conversion, switch to using integral values
for timestamps.
After the switch, printing the time difference shows the values as
expected.

Bug: 240168494
Test: printed diffs of timestamps inside palm rejection code
Test: atest inputflinger_tests
Change-Id: If6af45ba7740de4d70a300a45842bd3d6a069f40