Add a debug property to enable Skia systrace

Add a "debug.hwui.skia_atrace_enabled" property that allows to
record a systrace with skia drawing ops. App need to be restarted
after setting the new propety to true.

Test: collected skia trace with systemui
Change-Id: I2d1c5911714bfc911894f633ec2df2f6bb953b48