stagefright: enable native handles for secure buffers

- rename IOMX::allocateBuffer to allocateSecureBuffer as ACodec
  only uses allocateBuffer for secure compressed buffers.
- add argument to return native_handle if component supports it.
- rename IOMX::enableGraphicBuffers to enableNativeBuffers.
- add argument to select graphic vs. native handle mode
- request native handles for secure input buffers, but allow
  fallback

Bug: 26782004
Change-Id: Ide9d07f54d2e7e3e6a82dbca011f4db9a5630950