Handle RepoMappingManifest actions

These are essentially FileWrite actions, like SourceSymlinkManifest.

However, currently aquery appears to give non-deterministic results
for the file content. Emit an empty file for now to avoid the
non-determinism.

Fixes: 288292933
Bug: 287539324
Test: Presubmits
Change-Id: Ieaf8aff0d58d2d69087e4e34dc99074a015e700c