ICU-20769 refactor UResourceBundle.fResData & init_resb_result()

ICU-20769 refactor UResourceBundle.fResData & init_resb_result()
- replace UResourceBundle.fResData with access via fData
- change ResourceDataValue.resData into a pointer
- move C tests to C++ that stack-allocate now-C++ UResourceBundle
- init_resb_result():
  - remove redundant ResourceData parameter
  - simplify memory management, and various small simplifications
  - /LOCALE/path reuse valid locale, no ures_open()
  - pull alias-fetching code into separate function getAliasTargetAsResourceBundle()
  - `const char *` for keyPath pieces
  - s/noAlias/recursionDepth/
- getAllItemsWithFallback() set correct parentBundle.fValidLocaleDataEntry