ui: Refactor PinAndroidPerfMetrics into a serializable PinRequest IR

Split the plugin into three layers behind an unchanged external interface
(URL hash + command-palette prompt):

 - a serializable PinRequest IR (handlers/pinRequest.ts): a string-enum
   discriminant, one interface per variant, the union, and a light
   parsePinRequests() validator;
 - a Crystalball translation layer (handlers/handlerRegistry.ts +
   per-family translate* functions) mapping metric keys to PinRequest[];
 - an execution layer (handlers/executor.ts + per-family exec* functions)
   turning PinRequest[] into pinned tracks via a type-safe dispatch table.

MetricData/MetricHandler are unified into PinRequest; metricUtils.ts now
only holds JankType and expandProcessName. All SQL, handler ordering, and
dedup behavior are preserved. A new generic command
'com.android.PinAndroidPerfMetrics#pinRequests' accepts a PinRequest[]
argument so non-Crystalball clients can pin tracks directly.

TAG=agy
CONV=019d7f26-c882-47ce-8378-d03207d631d3

Change-Id: I7c50d80283544b6bcff3c9847fe440d5556c61fd