Check HIDL Discriminator for CSG Info

ART throws an exception if an unpopulated safe union element
is accessed at runtime, so instead we have to check the
hidl_discriminator to see whether the element is populated
before trying to access it (and thus we skip the null check).

Bug: 152486447
Test: atest testCellIdentityLte_1_5_CsgInfo \
        testCellIdentityWcdma_1_5_CsgInfo \
        testCellIdentityTdscdma_1_5_CsgInfo
Change-Id: I08dc0d2050a9f4d4b574178e1c68a23d015768e4