Do not modify passed in intent

If the caller wanted to use that intent object after passing it to this
method, it would be mutated in an unexpected way. Instead we create a
copy of the intent and broadcast that.

Bug: 144108192
Test: atest com.android.internal.telephony.CellBroadcastIntentsTest
Change-Id: I89e08b4a50beeb49410521baa9d449b2dc28ecd3