KeyboardInputMapper_test: send SYN_REPORTs

KeyboardInputMapper happens to be implemented in such a way that it acts
on key events immediately, without waiting for SYN_REPORTs, so many of
its tests don't send SYN_REPORTs at all. However, they really should,
both to avoid depending on that implementation detail of
KeyboardInputMapper, and to provide more realistic input.

Test: atest --host \
      frameworks/native/services/inputflinger/tests/KeyboardInputMapper_test.cpp
Bug: 245989146
Flag: TEST_ONLY
Change-Id: Ib7bee2d2203c5bdab67fdaf6df1e260ddb48dc95