Teach flutter logs to show only new logs

After this patch, the flutter logs command on Android shows only log messages
since the command was run. Previously, it showed historical logs, which was
confusing.

Fixes #2013

packages/flutter_tools/lib/src/android/android_device.dart

@@ -297,7 +297,7 @@ class AndroidDevice extends Device {
 
   // Return the most recent timestamp in the Android log. The format can be
   // passed to logcat's -T option.
-  String lastLogcatTimestamp() {
+  String get lastLogcatTimestamp {
     String output = runCheckedSync(adbCommandForDevice(<String>[
       '-s', id, 'logcat', '-v', 'time', '-t', '1'
     ]));
@@ -310,7 +310,7 @@ class AndroidDevice extends Device {
   Future<String> stopTracing(AndroidApk apk, { String outPath }) async {
     // Workaround for logcat -c not always working:
     // http://stackoverflow.com/questions/25645012/logcat-on-android-l-not-clearing-after-unplugging-and-reconnecting
-    String beforeStop = lastLogcatTimestamp();
+    String beforeStop = lastLogcatTimestamp;
     runCheckedSync(adbCommandForDevice(<String>[
       'shell',
       'am',
@@ -472,6 +472,8 @@ class _AdbLogReader extends DeviceLogReader {
       'logcat',
       '-v',
       'tag', // Only log the tag and the message
+      '-T',
+      device.lastLogcatTimestamp,
       '-s',
       'flutter:V',
       'ActivityManager:W',