Grant wmtrace access to platform_app:systemui

Grant read/write access to anyone for /data/misc/wmtrace folder on
debuggable builds, it's further protected by the selinux policy.

This is to allow systemui process to write proto logs to the same folder
on device as WindowManager, both can contribute to the transitions like
PiP, Split-Screen and etc.

Bug: 251513116
Test: adb shell dumpsys activity service SystemUIService \
      WMShell protolog [start | stop]
Change-Id: Ice57efa17c61d132b02c0a11a762c24d772bd90a