Remove stride from VTS classes

Stride must be retrieved from the graphic buffer immediately prior to
CPU access. The previous code was tripped up by an error-prone situation
where stride was never initialized, causing UB.

Also this patch sneaks in a couple of const modifiers for methods that
pass-by-ref.

Bug: 213493262
Test: VtsHalGraphicsComposer3_TargetTest
Change-Id: Ic601cca128b71d36876ed25bd55140b02cb0ad0f