Add javadoc to system api OpEventProxyInfo.getDeviceId()

According to the feedback from API council, add javadoc to getDeviceId()
to explain what the method is. The javadoc also matches the format of
existing docs of other getters.

Bug: 341319360
Test: only javadoc change.
Change-Id: I461b95cf66a807fa5239f93d258366df4203fd11