libcutils: add support for app tracing

This change adds support for tracing using the ATRACE_TAG_APP tag.  This tag is
enabled only for processes in which the /proc/<pid>/cmdline value appears in
the comma-separated list of such values in the debug.atrace.app_cmdlines system
property.  It is also only enabled if either the ro.debuggable system property
is set to 1 or the application has declared itself debuggable in its manifest,
which gets reported to libcutils by the framework.

Change-Id: Ic94ba55f4e70a9cf994056acbf151e523428b65d