Add incStrong and decStrong methods to android_native_{window,buffer}_t.

This change adds the methods necessary to use sp<> to handle refcounting
android_native_window_t and android_native_buffer_t.  The new methods forward
the refcounting operations to the corresponding android_native_base_t
functions.

Change-Id: I7de8e262728e439bc1efdf69374a2a9f6f432ced