Added unit test for MobileNetworkImeiPreferenceController visibility

The new test checks whether the item is available when the
SubscriptionUtil.isSimHardwareVisible() method returns true, i.e. if
the config_show_sim_info boolean flag is true on non-test runtime.

The default visibility status for all tests is defined in the test
setup() method.

Bug: 395714454
Flag: EXEMPT test only
Test: atest MobileNetworkImeiPreferenceControllerTest

Change-Id: Iaa8be041f0299ac36d532ed68fdc9c30e5690b11