Do not send multiple dereg indications when we switch SMS role

When we switch the SMS role, we get the following events:
A -> "" and then " " -> B. In order to not send multiple dereg
events when this occurs, only send a dereg event when the new
cached role package name is non-empty.

Bug: 192611985
Test: atest CtsTelephonyTestCases
Change-Id: I43898e0664f030069f32cf7b2f1c9267cf7a1d33
1 file changed