build: add a few proguard rules [1/3]

- Introduce a new directory for some specific proguard rules.

kotlin.flags:
- Remove DebugMetadata
- Don't warn about missing non-runtime annotations

Test: build AOSP & test on internal
Bug: 309023911
Change-Id: Iee740b61a2afeba3482ff4e1f8213bd4cf46174a