Fix One Euro filter's units of computation

Changed the units that the One Euro filter uses to compute the filtered
coordinates. This was causing a crash because if two timestamps were
sufficiently close to each other, by the time of implicitly converting
from nanoseconds to seconds, they were considered equal. This led to a
zero division when calculating the sampling frequency. Now, everything
is handled in the scale of nanoseconds, and conversion are done if and
only if they're necessary.

Bug: 297226446
Flag: EXEMPT bugfix
Test: TEST=libinput_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: I7fced6db447074cccb3d938eb9dc7a9707433f53