Rewrite concurrency logic in SurfaceFlinger PowerAdvisor

This patch aims to make concurrency safer by making sure all checks to
"hint session running" use mHintSession directly rather than a proxy
variable, and lock it. This requires all access to
"ensurePowerHintSessionRunning" to be locked, which requires refactoring
some methods, but should make the class much safer. This patch also aims
to ensure the session pointer is being directly checked for validity,
rather than relying on PowerHAL to always pass valid sessions.

Bug: 308030899
Test: atest libsurfaceflinger_unittest:PowerAdvisorTest
Change-Id: I0ebe55ed3c62c57cfd6fb94280beff4b9e8fcc13