teeui::str: replace static constexpr with inline constexpr

Fixes these build failures with clang-r547379:

system/teeui/libteeui/include/teeui/utils.h:182:13: error: explicit specialization cannot have a storage class [-Werror,-Wexplicit-specialization-storage-class]
  182 | template <> static constexpr const char* str<px> = "px";
      |             ^~~~~~
system/teeui/libteeui/include/teeui/utils.h:183:13: error: explicit specialization cannot have a storage class [-Werror,-Wexplicit-specialization-storage-class]
  183 | template <> static constexpr const char* str<dp> = "dp";
      |             ^~~~~~
system/teeui/libteeui/include/teeui/utils.h:184:13: error: explicit specialization cannot have a storage class [-Werror,-Wexplicit-specialization-storage-class]
  184 | template <> static constexpr const char* str<mm> = "mm";
      |             ^~~~~~

Bug: http://b/390013739
Test: m libteeui_hal_support
Change-Id: I9bb04f0ded9dd94b171fa36ca7d00c432b491783