Add AccessibilityTracing and trace AccessibilityController.

The tracing is for a11y engineering team use for debugging purpose. It
is disabled by default and can be enabled only by shell command. The cl
for the shell command change will be submitted after this cl. See "Test"
section below for the command details.
Expose interface through WindowManagerInternal to allow a11y use the trace feature.

Bug: 157601519
Test: adb shell cmd accessibility start-trace
      adb shell cmd accessibility stop-trace
Change-Id: I4cdb8798b29cd4a246db71d8cb17fe2c4429cd51