file_contexts

- Allow system_server to create and write to /data/misc/wmtrace/*
- Allow surfaceflinger to create and write files from /data/misc/wmtrace/*
- Allow dumpstate to read files from /data/misc/wmtrace/*
permissions are restricted to userdebug or eng builds

Bug: 64831661

Test: adb shell cmd window tracing start && adb shell cmd window tracing stop
Test: adb shell su root service call SurfaceFlinger 1025 i32 1 >/dev/null && adb shell su root service call SurfaceFlinger 1025 i32 0 >/dev/null
Test: adb bugreport ~/tmp.zip && adb shell su root dmesg | grep 'avc: '

Change-Id: I0b15166560739d73d7749201f3ad197dbcf5791c