[A11y] Adds ability for magnifier to follow mouse and stylus

Lets magnifier follow mouse and stylus hover moves and mouse
moves, when flag is enabled. No behavior change when flag
is disabled. Mouse following is "continuous" mode, where the
mouse traverses across the screen as the viewport moves from
edge to edge.

Bug: b/354696546
Test: Manual in emulator; new automated tests
Flag: com.android.server.accessibility.enable_magnification_follows_mouse
Change-Id: Ic6080224e0ae1bfb5e8441820612be68842fc1ea