Allow custom density setting

Use system property persist.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.

Change-Id: I8d26405d5d33bdf2890a0e9f67f113a4dc3e763b

Conflicts:
	core/java/android/content/res/Resources.java