Changed target path for transaction writing.

Previously, transactions were written into a directory (data/SurfaceTrace.dat)
that required SELinux write permissions. Changing it to the new path now allows for it to
be accessed with no permission changes needed.

Test: adb shell su root service call SurfaceFlinger 1020 i32 1 then
verify that the transaction file is in
/data/misc/wmtrace/transaction_trace.pb

Change-Id: I1fc30bca93e7f2a064db856be223ddd64b53fbda