Update Cipher change markers.

Mostly adds a note documenting that we changed much of the provider
selection code.  Also marks some more important/less obviously-changed
chunks of code explicitly.

Reverts a few unnecessary diffs from upstream 8u121-b13, and readds
(commented out) two large blocks of code that were removed.

Test: cts -m CtsLibcoreTestCases -t libcore.javax.crypto.CipherTest
Bug: 38192166
Change-Id: I47769203beb41a8308d7cbd5d1be1d1a12a4c0ac