Add the DPAD device flag when injecting accessibility DPAD events.

For the global DPAD actions GLOBAL_ACTION_DPAD_UP/DOWN/LEFT/RIGHT/CENTER, set the event source to both SOURCE_KEYBOARD and SOURCE_DPAD to match the event we would get with a real DPAD of a TV remote control.

Bug: b/273880258
Test: Locally tested on adt4-userdebug with the same sample accessibility service used in the video attached to b/273880258.
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4df28baa74d2dbde690d5357f1807e59f4eadd6d)
Merged-In: Icc194345bca7220927f98ae67379c27a9e8c32ba
Change-Id: Icc194345bca7220927f98ae67379c27a9e8c32ba