Sends ACTION_CANCEL before the very first touch exploration HOVER event.

This resets the InputDispatcher state in case the user had a pointer
down (and kept it down) before enabling TalkBack or while booting up
the device before TalkBack has started up.

Fix: 364408887
Test: atest TouchExplorerTest
Test: touch and hold on the screen;
      start TalkBack with a shortcut or adb shell;
      lift your finger off the screen;
      touch the screen again, and observe that touch exploration
      functions properly.
Flag: com.android.server.accessibility.reset_input_dispatcher_before_first_touch_exploration
Change-Id: I1c2bdb7b3d32cd124257e60e8c91c073c3040f50