libbinder_ndk: mark 29 API on old plat funcs

Didn't seem necessary at the time, esp. for functions that were for the
platform and introduced in the original version of the library. Though,
adding now.

Fixes: 185256359
Test: N/A
Change-Id: I7d94cae3f379783fe2e92f2c2422b0e14f00310b