scripts: Fix safe_struct Android versioning issue
scripts: Fix safe_struct Android versioning issue

The VK_ANDROID_external_memory_android_hardware_buffer extension uses
an android structure that is undefined before android 'O'. Worked around
this issue by defining a dummy structure for earlier android OS versions.

Change-Id: If515920d3b1707cbeb6a0377827c680d93d516ff

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools