Update javadoc for DISALLOW_SIM_GLOBALLY

DISALLOW_SIM_GLOBALLY is only enforced in EuiccController.java to disallow download of new non-managed embedded SIMs when this restriction is set.
Updating the documentation to reflect that we are not disabling physical SIM or affecting any existing embedded SIM already present on the device.

Bug: 295301164
Test: NA
Change-Id: I51a2660fa39211e5428fa491f95ba673d19a7d50