Use define for drm property keys instead of defining as char pointers

When defined as char pointers, every compilation unit that includes
the NdkMediaDrm.h header defines a new copy of the same global
variables - leading to linker errors due to multiple definitions
if more than one file includes it.

By using a define, they don't generate any symbols. The NdkMediaFormat.h
header uses extern const char* for a similar set of keys, but that
can't be added after the actual platform has shipped. The other
alternative would be to declare them as static const char*, which
wouldn't generate global symbols, but which could trigger warnings
about unused static symbols instead.

Change-Id: I14ca81d94309b7f437b3bc144920c48a8b3f0261