Systrace doc updates

Update to reflect new user interface and command line options
in Android 4.3+. Also add section on using application-level
tracing.

Fixes for bugs:
b/7306847
b/10113361
b/9425766
b/7306847
b/9262485

Change-Id: Ia1c2a54fa2edc12cd68da27055aebe7ec47df745