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.

Change-Id: I8d26405d5d33bdf2890a0e9f67f113a4dc3e763b
settings: convenience functions for booleans

Change-Id: I27b66575d5cda575bbc38737697951520327d31a