Add CellBroadcastIntentsTest

testIntentIsNotMutated verifies that when the caller passes in an
intent, the method does not modify that intent's flags.

Bug: 144108192
Test: atest com.android.internal.telephony.CellBroadcastIntentsTest
Change-Id: I6226780bda6bff48141810e6d58f6d647963b5f3