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