Use std::vector for pointers in InputState

This simplifies calls to other APIs, because we can now pass the vector
by ref instead of having to separately store the pointerCount.

Bug: 211379801
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: I0f8af524afe99fee93b937890754d9676f8af5e7