String on Toast notification does not aligned to center

If length of the string being showed on Toast is short (less than 8 chars),
it is aligned to left rather than to center.
Added attribute android:layout_gravity="center_horizontal" on the TextView where Toast's message are shown to fix issue.

Change-Id: I77669fd8a11d1ec8f6b082348eb818671ce158bb
Signed-off-by: Taeho Kim <jyte82@gmail.com>

core/res/res/layout/transient_notification.xml

@@ -29,6 +29,7 @@
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_weight="1"
+        android:layout_gravity="center_horizontal"
         android:textAppearance="@style/TextAppearance.Small"
         android:textColor="@color/bright_foreground_dark"
         android:shadowColor="#BB000000"