Enable build flags in protobuf

If RELEASE_BUILD_FLAGS_IN_PROTOBUF is False, then the results from
out/release-config are ignored and we do the old processing.

The following additional logic all goes away once the migration to protobuf
is finished.

- If we found .textproto files that don't have corresponding .mk files,
  we require protobuf.
- If we found .mk files that don't have corresponding .textproto files,
  we require legacy (make) processing.
- If PRODUCT_RELEASE_CONFIG_MAPS specifies .textproto files, we require
  protobuf.

In order to further isolate migration, the choice can be determined on a
release-config by release-config basis.  in each release config
directory, we look for:
 - `build_config/DEFAULT=proto`
 - `build_config/DEFAULT=make`
 - `build_config/${TARGET_RELEASE}=proto`
 - `build_config/${TARGET_RELEASE}=make`
The last such file found determines which code path is used.

Bug: 328495189
Test: manual, TH
Change-Id: I1d84db76b157082fc7db1b3cb7c9afe8cdf46cbf