bring back DPI changer

originally part of DPI Changer + Tablet UI commit on ICS by @sethyx and @dagr8
cherry-picked and adapted for JB-MR1 by @championswimmer and @kecinzer

PS4:Added situation that values don't exists in build.prop
PS5: compare length() == 0 (instead of check for NULL)

PS6: we are keeping the long procedure intact because we never know
if some day play store starts checking persis.lcd_density too,
so let's keep the compatibility workarounds,
even though atm they are not needed

PS7: rebase
PS8: set author to Roman
PS9: set xxhdpi to 480dpi (as per S4 and ZL stock build.prop)

Change-Id: I3e3a63eb64b0f107c1fad0f2fd8bb7c7f6e6e227
Signed-off-by: ChampionSwimmer <android@championswimmer.tk>