WifiEnterpriseConfig#getDecoratedIdentityPrefix @Nullable

Following up on API council feedback, modify
WifiEnterpriseConfig#getDecoratedIdentityPrefix to return a the
actual value of the decorated prefix member, including a null
value, to be in parallel with the PasspointConfiguration API.

Bug: 183605102
Test: atest WifiEnterpriseConfigTest
Test: atest com.android.server.wifi
Change-Id: Ic67edd503a2bcbbaf6e40a918728563a45a4a1d9