Nearby: Turn off proguard optimization of proto classes.

Add a proguard rule to prevent proguard from stripping out
fast-pair-lite-protos classes/fields.

Fix: 218607668
Test: atest CtsNearbyFastPairTestCases and manual steps in b/218607668#comment2
Ignore-AOSP-First: Nearby is not yet in AOSP. This introduces minimal
                   unavoidable merge conflicts until it is merged there.
Change-Id: I947c9e3b6ebc232e230782f6345c5cb6893138e6
2 files changed