Optimise input stack for better performance using buffers.

Also fixed some minor bugs on other classes.

I did some adhoc benchmarking by running
IntegrityFileManagerTest#testGetRules_indexedForManyRules. Previously it took
282 ms to load 270 rules from 5070 rules. Now it takes 12 ms. More comprehensive
benchmark will come later.

Change-Id: Ic3b239b54fdd9e422ada8994afaa666b0f98a62a
Bug: 143697198
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/integrity