Faster unweighted 2nd degree least squares algo.

Speed up the default velocity computation that uses
unweighted second degree least squares approach.
Only calculate the linear coefficient, single loop
with O(n) complexity. About 2x speedup.

Test: dumped the original and the new values while
flinging settings. Observed nearly identical
velocity results (less than 0.1% difference).
Also ran CTS test:
tools/cts-tradefed run cts -t android.view.cts.VelocityTrackerTest -m
CtsViewTestCases --skip-system-status-check
com.android.compatibility.common.tradefed.targetprep.NetworkConnectivityChecker

Change-Id: I7ddbd85a092f44e8d4b5f109c386af5dd589d249