[idmap] Cache target apks as they are often reused

idmap2d is being often called for the same targets, e.g. system
apps. Caching those apks makes idmap verification and creation
much faster e.g. for user switching

Test: build + boot + UTs
Bug: 271904589
Change-Id: Ib6f7af385c2389b50d5c74aa08b4bab290580809