Fix __ANDROID_API__ guards for libnativewindow

Partial revert of:
 - "Add __INTRODUCED_IN to <android/native_window.h>."
   commit a2c9607eab7214db97e4a63fb18154d4dfd95284.
 - "Add __INTRODUCED_IN annotations to <android/hardware_buffer.h>."
   commit 493f4a23bf586bf1bcd5e9bc73ed3c94852fc199

Most of these APIs were available via libandroid.so in API 9.

Bug: http://b/111668906
Bug: http://b/111660252
Test: m
Change-Id: I40874c4cd3413abf26f74f21caa5fb845fab721b
Merged-In: I40874c4cd3413abf26f74f21caa5fb845fab721b
2 files changed