vendor/omni: Add support for low fps boot animation. 

Some devices can not support over 100+ fps, leaving too 
much black screen time during boot. 

To use, add flag 'USE_LOWFPS_BOOTANI := true' to your 
omni_<device>.mk file, ahead of the call to 
vendor/omni/config/common.mk (or common_tablet.mk). 

This was tested on clean builds for tf101 and with flag 
(and this patch) the low fps boot animation was pulled in 
and without the flag the normal boot animation was pulled in. 
Should work for all devices.

Change-Id: I461381093ca5783a074f35bc5b8aa852c5607d2f