change CryptoPluginManager to be instantiated in the module scope

Fixes an issue where SecretStore was repeatedly creating new instances
of the stevedore extension manager, which caused issues with the loaded
plugins.

Closes-Bug: 1358386
Change-Id: Icfafce0ec6fbacc2c39e23ea31f200a56197b7cb
Merge "change CryptoPluginManager to be instantiated in the module scope"