This corrects the button state emitted on synthetic pointer move and
hover events generated by the engine.

When an embedder notifies the engine of a pointer up or down event, and
the pointer's position has changed since the last move or hover event,
`PointerDataPacketConverter` generates a synthetic move or hover to
notify the framework of the change in position. In these cases, the
current event from the embedder contains the new button state *after*
the pointer up/down event, but the move/hover needs to be synthesized
such that it occurs *before* the pointer up/down, with the previous
button state.

This patch stores the button state after each pointer down, up, move, or
hover event such that it can be used by the next event if a synthetic
event must be issued.

The bug in the previous logic was revealed by the release of macOS 11
(Big Sur), which appears to issue move events between mouse down and
mouse up, which did not use to be the case.

This fixes https://github.com/flutter/flutter/issues/64961, which is the
desktop-specific tracking bug for the more general Big Sur mouse click
umbrella issue https://github.com/flutter/flutter/issues/71190.