LSC: Migrate from //strings/join.h to //third_party/absl/strings/str_join.h
LSC: Migrate from //strings/join.h to //third_party/absl/strings/str_join.h

Migrate strings::Join() (and the related symbols that once lived in
//strings/join.h) to absl::StrJoin() (and its related symbols that now
live in //third_party/absl/strings/str_join.h).

This CL adds the str_join.h header where necessary, and may cause some
re-ordering of #include files, courtesy of clang-format.

See b/68809852 and go/lsc-absl-gplc for more information.

Tested:
    TAP sample presubmit queue
    http://test/OCL:177448330:BASE:177448432:1512053798571:3fd09170

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177715343

GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java,GoogleCloudPlatform/cloud-debug-java