libgui: Add simpler methods to ConsumerBase and BufferItemConsumer.

Now we can detach a buffer directly.

BYPASS_IGBP_IGBC_API_REASON=warren buffers

Bug: 340933754
Flag: com.android.graphics.libgui.flags.wb_platform_api_improvements
Test: new test in libgui_test
Change-Id: Ib14e645a04b21b3233522d2d20ee5aaa09fe563c
5 files changed