logd: add getEventTag command and service

Will register a new event tag by name and format, and return an
event-log-tags format response with the newly allocated tag.
If format is not specified, then nothing will be recorded, but
a pre-existing named entry will be listed. If name and format are
not specified, list all dynamic entries. If name=* list all
event log tag entries.

Stickiness through logd crash will be managed with the tmpfs file
/dev/event-log-tags and through a reboot with add_tag entries in
the pmsg last logcat event log. On debug builds we retain a
/data/misc/logd/event-log-tags file that aids stickiness and that
can be picked up by the bugreport.

If we detect truncation damage to /dev/event-log-tags, or to
/data/misc/logd/event-log-tags, rebuild file with a new first line
signature incorporating the time so mmap'd readers of the file can
detect the possible change in shape and order.

Manual testing:

Make sure nc (netcat) is built for the target platform on the host:
$ m nc

Then the following can be used to issue a request on the platform:
$ echo -n 'getEventTag name=<name> format="<format>"\0EXIT\0' |
> nc -U /dev/socket/logd

Test: gTest logd-unit-test --gtest_filter=getEventTag*
Bug: 31456426
Change-Id: I5dacc5f84a24d52dae09cca5ee1a3a9f9207f06d
diff --git a/logd/LogStatistics.cpp b/logd/LogStatistics.cpp
index 273150e..7e0a6b7 100644
--- a/logd/LogStatistics.cpp
+++ b/logd/LogStatistics.cpp
@@ -452,12 +452,11 @@
         name = android::base::StringPrintf("%7u/%u",
                                            getKey(), uid);
     }
-    size_t len = 0;
-    const char *nameTmp = getName(len);
+    const char *nameTmp = getName();
     if (nameTmp) {
         name += android::base::StringPrintf(
-            "%*s%.*s", (int)std::max(14 - name.length(), (size_t)1),
-            "", (int)len, nameTmp);
+            "%*s%s", (int)std::max(14 - name.length(), (size_t)1),
+            "", nameTmp);
     }
 
     std::string size = android::base::StringPrintf("%zu",