Initialize pointerCapture

Currently, pointerCapture variable is not initialized, so it will be set
to whatever is currently occupying that address.

This causes flakiness of the input native tests,
because in this test case, FakeInputReaderPolicy does not do any special
initialization for InputReaderConfiguration.

Bug: 117116942
Test: asan-built inputflinger tests
Change-Id: Id9a6b2f57bf3722e9d92ba1408468cf3bdf721b2