InputReader: Use shared keyboard source for all key events

When multiple EventHub devices are merged into a single InputDevice,
it's possible that there are more than one KeyboardInputMappers created
for the device. In this case, each mapper could be configured with a
different source. This can lead to situations where a device generates
key events that have different sources, depending on the mapper from
which it originated.

To make sure all key events use a consistent source for each
InputDevice, use the shared keyboard source when generating events from
a KeyboardInputMapper.

Bug: 354270482
Test: atest inputflinger_tests
Flag: EXEMPT bugfix
Change-Id: I586424b5e8d31d17cbe635d9f91a889aee906d40