Make custom ShadowInputDevice#getDeviceIds return ids from deviceMap

In the past, besides adding the device to ShadowInputDevice, the caller
also need to change the static deviceIds. This results duplicate efforts
and would be error pront when the deviceIds doesn't match the available
devices' ids.

Changing #getDeviceIds returns ids from the deviceMap matchs the
InputDevice#getDeviceIds API while also reducing the duplicate efforts
on creating deviceIds arrays.

Test: atest SettingsRoboTests
Flag: EXEMPT test only
Change-Id: I6c8c9baa26c6826fc3433b8024165df695c008bf