Fix GCM IV length

Earlier versions of Keystore generated and stored a 16 byte IV, but used
only the first 12 bytes. Simply generate and store a 12 byte IV, while
processing the longer IVs as before.

Bug: 163866361
Test: keystore2_test
Change-Id: I6b30ad3f41515ce224f282fe555a03580ee7c214
2 files changed