themechooser: decode inSampleSize prior to load wallpaper bitmap

Decode inSampleSize to avoid OOM exceptions loading the wallpaper bitmaps

JIRA: CYAN-4795
https://jira.cyanogenmod.org/browse/CYAN-4795
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>

Change-Id: I2cbc809d5fe6b119f0715db40d2f175a05c9b913