[SNAPI] Fix JSON field naming mismatch in Soong API database queries

The GetModuleByInstallPath RPC was returning empty results even when
valid modules existed in the database.

The issue was caused by an inconsistency between the JSON field names
stored in SQLite and the path expressions used in SQL queries:
1. By default, com.google.protobuf.util.JsonFormat converts snake_case
   fields (e.g., install_files) into camelCase (e.g., installFiles)
   during the translation from Protobuf to JSON.
2. The SQLite json_each function is case-sensitive. It was searching
   for $.install_files, but the actual keys in the database were
   installFiles, resulting in zero matches.

Solution:
1. Loader Fix: Configured JsonFormat.Printer in SoongApiLoader.java with
   .preservingProtoFieldNames() to ensure JSON output maintains the
   original snake_case naming defined in the .proto file.
2. DAO Optimization: Refactored the SQL query in SoongApiDao.java to
   use an EXISTS clause with the correct $.install_files key. The
   EXISTS approach improves performance by short-circuiting once a match
   is found and ensures unique result sets.
3. Test Enhancement: Updated SoongApiLoaderTest.java to explicitly
   verify that the ingested JSON uses snake_case naming and does not
   regress to camelCase.

Bug: 471114931
Test: atest soong_api_tests --host
Change-Id: I09b70f0e6ed6ad80b4ea4a4d6797522bb198dba9