Allow custom density setting

Use system property persist.sys.lcd_density to set custom density.  The
custom setting affects the entire UI.  It is independent of
ro.sf.lcd_density and has no effect on play store compatibility.

Code distilled from PA project via PAC project.

Hmm... Previous thing for 'density' doesn't work at all. Let's try this one. Thx to AOKP Team!

Change-Id: I8d26405d5d33bdf2890a0e9f67f113a4dc3e763b

Conflicts:
	core/java/android/util/DisplayMetrics.java