Fix output stream creation in layertracegenerator

layertracegenerator creates the output layers trace file specifying
std::ios::app (append mode). Meaning, if an output file already exists
layertracegenerator appends the new layers trace to it.

This commit solves the issue replacing std::ios::app with std::ios::out.

Bug: 301539901
Test: execute multiple times \
    adb shell layertracegenerator transaction.winscope test.winscope
    and make sure the size of test.winscope remains constant \
    and doesn't increase after each execution of layertracegenerator
Change-Id: I23bf4e689cee450b055a0451aa0301ce34131739