libutils: liblog export headers

- make liblog dependency for all library builds (this is required
    for files like String8.cpp)
- export liblog headers (because they are used in many header files).

Test: less libraries fail with BOARD_VNDK_VERSION := current
Merged-In: Iecb9cd00deb3a9056ea63c4a087afdb80a51a2b8
Change-Id: Iecb9cd00deb3a9056ea63c4a087afdb80a51a2b8