use frozensets to represent APK cert sets
use frozensets to represent APK cert sets

Change-Id: Ic92c83be0c3f6a76bf1bd21f0216cd1b87586959

RR-msm7x30/platform_manifest,xorware/android_build,hagar006/platform_manifest,pranav01/platform_manifest,Hybrid-Power/startsync,ND-3500/platform_manifest,xorware/android_build,xorware/android_build,xorware/android_build,knone1/platform_manifest,xorware/android_build,see4ri/lolili,fallouttester/platform_manifest