Dynamically add STYLUS source for multi-touch devices

Since we cannot tell definitively whether a multi-touch device that
supports MT_TOOL_TYPE will ever report MT_TOOL_PEN, we default to
configuring the source of such a device not not use SOURCE_STYLUS, and
it to events dynamically if it ever produces a stylus event.

Bug: 246991366
Test: atest inputflinger_tests
Change-Id: I70cc15f8f354e3fc876024d194a6f320fabd6d2c