Migrate mStashedInApp to FLAGS_STASHED_IN_APP

This is to prepare for different flags that could cause taskbar to be stashed in an app without the user explicitly long pressing to stash.

Test: run wwdebug and wwlogcat, ensure still get logs for long press stash events; other interactions like clipping tasks to above unstashed taskbar still work as before
Bug: 190192993
Bug: 193937948
Change-Id: I90846e650a438e03bdcfdf9c4bf919e19cc5abb3