cameraservice: Add metrics for extension sessions

We want to log camera extension usage in cameraservice. This CL adds
the scaffolding for it. Specifically, it does the following:

  - Updates CameraSessionStats.java to match that from
    CameraSessionStats.h.
  - Updates CameraServiceProxy to log CameraExtensionSessionStats
    along with CameraSessionStats.
  - Adds ExtensionSessionStatsAggregator responsible for collecting and
    sending extension stats to cameraservice.
  - Updates CameraExtensionSessions to use
    ExtensionSessionStatsAggregator.

Bug: 261470491
Test: Only adds metrics logging. No functional change.
      `atest CtsCameraTestCases` pass.
      `statsd_testdrive 227` confirms that extension metrics are logged
          correctly.
Change-Id: I40128f41c25daca8b4cc87a862f67990f8928caf