Refactor HintSessionWrapper and move into CanvasContext

 * Move HintSessionWrapper into its own file
 * Refactor HintSessionWrapper initialization and lifecycle
 * Move HintSessionWrapper instance into CanvasContext
 * Have HintSessionWrapper do its own validity checks
 * Have HintSessionWrapper implicitly initialize when called

Bug: b/260267128
Bug: b/260224719
Test: manual
Change-Id: I69f89bb5ef601d1b5d9fca1a5da62256f2556da8