Use vector instead of VLA in solveLeastSquares

For simpler tracking of the sizes of the arrays, use vector instead of
VLA. Also, VLA is not part of c++ standard. It's also being removed from
the kernel code.

Bug: 167946721
Test: atest libinput_tests
Change-Id: I03e5ad934bc3d9f451c76d0415c6a1254ec0053a