libstagefright: Support for legacy camera/encoder buffers

Since Android 7.0, the default way of passing buffers
between camera HAL, encoder and framework is via
Android Native Window (ANW) buffers.

Pre 7.0 devices did this using buffer_handle_t buffers
allocated by gralloc HAL.
Since many devices do not have the source code for their
camera HALs or the encoder libraries available, introduce
this hack.

Change-Id: I202051ab5d7273a33be0aa32b0bfdbbb1f53693f