Dynamically add STYLUS source for multi-touch devices - part 2

This is a follow-up to I70cc15f8f354e3fc876024d194a6f320fabd6d2c.

The previous change added SOURCE_STYLUS to events dynamically if they
came from a stylus tool type. In this change, we instead dynamically add
SOURCE_STYLUS to the InputDevice whenever it produces an MT_TOOL_PEN
event. This means all the events generated by that device will have
SOURCE_STYLUS added to it from that point onwards.

Bug: 246991366
Test: atest inputflinger_tests
Change-Id: I61963563eb4cdd41957247db100fcb87cc1b278b