Add APIs for application-generated systrace events

This change adds new APIs to enable applications to generate custom Systrace
begin/end events.  Application-generated events use the ATRACE_APP_TAG tag,
which is enabled only if either the application has declared itself debuggable
in its manifest or ro.debuggable is set to 1 on the device.

Change-Id: I311d09e2e6ed1a30f5ffa84907f250e11cc0d48d