Respond to various API council feedback.

Handle many simple, smaller changes in a single CL.  Hide
CPC.closeQuietly(), now that it implements AutoCloseable.  Add more
details to CR.set/getCache() docs.  Add many @Nullable/@NonNull
annotations.

Bug: 124507578, 124447751, 124302519, 123697622
Bug: 123661322, 122887179, 122528742, 122527812, 116224797
Test: manual
Change-Id: I78faf4c51d7818b60c0c12699e0add71f1f548dd