Improve content capture trace and prevent redundant works

1. Only start trace session when content capture is enabled.
2. Only start notifyContentCaptureEvents loop when content capture is
   enabled.
3. Make sure FLAG_SECURE will not have effect when the session is
   already disabled by the app. Otherwise, the default absense of
   FLAG_SECURE will temporarily override the disabling state by app.
   This would make ViewRootImpl cache the incorrect state during startup
   and run some redundant works.

BUG: 295199827
Test: refactoring CL. Existing unit tests still pass.
Change-Id: I422c7f9af6ce296cdb26d5575445f50fc298de4f