Add --trace_folder to aapt2

Add a tracing API and instrument key functions in order to profile
aapt2 bottleneck. The API allows to generate systrace fragment files.

Impact on performance is neglibible with each Trace requiring less
than 1us and the final Flush operation at the end of a command
requiring around 40us.

Bug: None
Test: None
Change-Id: I51b564d3694e9384679f43b878b32295527dddf6