Fix anonymous struct and union warnings

gui/BufferItem.h and android/sensor.h uses anymous structs and nested
anonymous unions, which are GNU extensions.  sensor.h uses them as part
of its API, so disable the warnings in libgui, the only module that
tries to use it with -Weverything.  BufferItem.h only uses the unioned
fields inside libgui, remove the union and do the 64-bit to 32-bit
slicing manually so libvulkan doesn't need the warnings disabled.

Bug: 31752268
Test: m -j
Change-Id: I92d59b1202f4d6e5419edaa6d27b6e1c50ac0042