Stylus fusion: Use SOURCE_BLUETOOTH_STYLUS dynamically for events

Previously, whenever an external stylus was connected, the source for
all touch devices (touchscreens, styluses, drawing tablets, etc.) would
change to include SOURCE_BLUETOOTH_STYLUS. This meant that all events
produced by these devices would also include SOURCE_BLUETOOTH_STYLUS,
even if the event was unaltered by stylus fusion.

In this CL, we introduce a way to dynamically add
SOURCE_BLUETOOTH_STYLUS to event streams that are a product of stylus
fusion.

The problem we are trying to solve is to be able to differentiate a
normal event stream from a stylus vs. an event stream from a stylus or
touchscreen that has additional information fused from a bluetooth
device. Previously, both such streams would have
SOURCE_BLUETOOTH_STYLUS, whereas now, only the latter case would use
that source.

Bug: 300473125
Test: atest inputflinger_test
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:b08a0e84208886a3192803745b48c0d348578e31)
Merged-In: I3526d42df68bc899c8e9a0e5ad69c95864f4c325
Change-Id: I3526d42df68bc899c8e9a0e5ad69c95864f4c325