Remove DetachedBufferHandle from libui

DetachedBufferHandle was a temporary measure introduced in Android P
to bridge BufferHub and GraphicBuffer. In Android Q however,
GraphicBuffer will be directly backed by BufferHub, thus
DetachedBufferHandle becomes redundant.

Also note that removing DetachedBufferHandle from libui should bare no
impact on vendors for two reasons:
1. BufferHub in P is only enabled for Daydream ready
devices (i.e. Pixel lines and VR AIO devices). No other vendors should
have BufferHub enabled.
2. DetachedBufferHandle.h was hidden from vndk and thus vendors
shouldn't get access to it anyway.

Bug: 117522732
Test: Build system
Change-Id: I3828eaa9499051e5ad5e4e270b5c26bae5f2c707