Cherry-pick: ICU-21247 Move #include "unicode/localpointer.h" into U_SHOW_CPLUSPLUS_API block

Upstream commit: https://github.com/unicode-org/icu/commit/7a57d299df350f1a175142a4079c39ca97f3391b

It removes localpointer.h in libandroidicu because they are C++ APIs.
Avoid including localpointer.h when generating new headers without U_SHOW_CPLUSPLUS_API flag

Bug: 160350521
Test: ./generate_libandroidicu.py
Change-Id: I386f9aaf868767a6511d30d6de16dc3b13661840
74 files changed