Prevent session destruction on CanvasContext::destroy()

While sessions not getting destructed on CanvasContext::destroy()
was initially suspected as a culprit for a memory leak, there are many
cases where CanvasContexts are repeatedly destroyed then reused. This
caused excessive re-creation and prevented hints from being sent, since
hints can't send while waiting for the session, causing regression. The
session will ultimately get destroyed in CanvasContext's destructor, so
leaving these sessions active for the lifecycle of CanvasContext is
fine, especially since the session is usually getting reset before new
work comes in anyway.

Bug: 297604399
Test: uibench runs of regressed metrics
Change-Id: I209e40764d5bbcde85d709bccbc41f94f9c2ab49
Merged-In: I209e40764d5bbcde85d709bccbc41f94f9c2ab49
(cherry picked from commit 9f3654e6)