Fix 5367082 correct assets for "next" and "previous" in lockscreen

Two issues found in the bug:
1/ The lockscreen layout for the transport controls was referring
 to the wrong framework icon: "previous" instead of "rew", and
 "next" instead of "ff".

2/ The framework icons for "previous" and "next" were wrong:
 "|<" should be "|<<" and ">|" should be ">>|". Copied the
 correct assets from the UX repository:
 btn_playback_ff_normal_holo_dark.png copied to ic_media_next.png
 btn_playback_rew_normal_holo_dark.png copied to ic_media_previous.png

 Note that those assets do NOT exist for ldpi

Change-Id: I9c92d468a984f47f8e8f8c23e00f85a3d03da2ba

core/res/res/layout/keyguard_transport_control.xml

@@ -63,7 +63,7 @@
                     android:layout_width="wrap_content"
                     android:layout_height="wrap_content"
                     android:layout_gravity="center"
-                    android:src="@drawable/ic_media_rew"
+                    android:src="@drawable/ic_media_previous"
                     android:clickable="true"
                     android:background="?android:attr/selectableItemBackground"
                     android:padding="10dip"
@@ -94,7 +94,7 @@
                     android:layout_height="wrap_content"
                     android:layout_gravity="center"
                     android:clickable="true"
-                    android:src="@drawable/ic_media_ff"
+                    android:src="@drawable/ic_media_next"
                     android:background="?android:attr/selectableItemBackground"
                     android:padding="10dip"
                     android:contentDescription="@string/lockscreen_transport_next_description"/>