Remove static modifier from KeySetManagerService issuedIds.

There is no notion of shared issuedIds across instances of KeySetManagerService,
of which there should only ever be one on the system.  This is particularly
troublesome for unit tests which rely on a clean slate for each
KeySetManagerService invocation.

Bug: 19530120
Change-Id: I41fbd5424650a74426c5aa1128ddfa6a230dcfa3