Record some logcat output with crashes, ANRs, etc..

Shelling out to logcat from the system server makes me queasy,
so this is turned off by default -- it must be enabled individually
for each error type (system_app_anr, etc) via a secure settings
value (which I plan to poke into from gservices for internal use).

Even when enabled, it happens in a side thread, unless the system
server is about to die anyway (system server restart).

Change-Id: Id6d88bcd78d3625f0364a5fe9c771046601a5a14
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index 873e1a6f..503d8b8d 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -2900,6 +2900,15 @@
          */
         public static final String DROPBOX_TAG_PREFIX =
                 "dropbox:";
+        /**
+         * Lines of logcat to include with system crash/ANR/etc. reports,
+         * as a prefix of the dropbox tag of the report type.
+         * For example, "logcat_for_system_server_anr" controls the lines
+         * of logcat captured with system server ANR reports.  0 to disable.
+         * @hide
+         */
+        public static final String ERROR_LOGCAT_PREFIX =
+                "logcat_for_";
 
 
         /**