Block touches when stylus is down

Since we haven't yet added simultaneous touch and stylus support, let's
improve the user experience by blocking all touch when the stylus is
down.

Bug: 210159205
Test: atest inputflinger_tests
Change-Id: Id6a6467d7feb7c7d91770ddbd63b92583832d504
(cherry picked from commit a3c8e519)