Add a debug flag to convert all touch pointers to stylus

When the sysprop `persist.debug.input.simulate_stylus_with_touch`
is set, all pointers reported by TouchInputMapper will be converted
into stylus pointers.

Changes to the flag will require a reboot to take effect.

Bug: 217741685
Test: manual
Change-Id: I7723be718354872509a9e85beda10acdbe25dd48