Keep track of pilfered pointer explicitly

The pointers that are getting pilfered were not being tracked explicitly
before. The behaviour relied on the window going away in order to reset
the "isPilfering" value to "false".

Now, however, some windows are getting persisted longer, because they
might receive a hover gesture that doesn't end (like a mouse that
someone stops using).

Therefore, we need to explicitly keep track of which pointers are being
pilfered. This is also in line with our eventual goal of explicit
per-window pointer management.

In the failing case, the windows was considered to be "pilfering". Since
the gesture monitors were receiving all of the pointers, the pointers
would no longer get dispatched to any windows underneath.

With this CL, we maintain the existing intended behaviours of pilfering,
while also expanding the support for multiple pilfered pointers.

In the added test case, there is a mouse interaction first, which
persists the gesture monitor window. Next, we pilfer a touch event,
which sets the "isPilfering" flag to ON. And finally, we inject the new
touch gesture. This gesture was (incorrectly) getting only sent to the
gesture monitor, because it kept maintaining the "isPilfering" status.

Bug: 266705421
Test: m inputflinger_tests && $ANDROID_HOST_OUT/nativetest64/inputflinger_tests/inputflinger_tests --gtest_filter="*MouseAndTouchWithSpyWindows*"
Change-Id: I6b72dc3e4de0e0ef09ccc469df0f703762c76fa6