[Pandora] Change has<Field> to get<Field>Case

Internal branches only support protobuf v3.9.1 which does not
provide has<Field> helper functions.

Bug: 245578454
Ignore-AOSP-First: cherry-pick from AOSP
Test: atest pts-bot
Change-Id: Ide921eba99f8c6caeaf7c8d2973bd8239336997c
Merged-In: Ide921eba99f8c6caeaf7c8d2973bd8239336997c