SF: clean up casting around histogram reporting

Takes advantage of the new binder Uint64Vector send/receive
capabilites to clean up a messy cast in the display histogram
functionality.

Fixes: 120504999
Test: boot
Test: libgui_test --gtest_filter="DisplayedContent*"

Change-Id: Ie8fe724d4d81999777e6ce1e8a610f55e0070d37