Refurbish granting mechanism

Keystore stores key blobs in with filenames that include the symbolic
name and the uid of the owner. This behaviour should have been
completely opaque to the user keystore. However, the granting mechanism,
by which an app can allow another app to use one of its keys, leaked the
internal structure in that the grantee had to specify the key name with
the granter's uid prefix in order to use the granted key. This in turn
collided with prefix handling in other parts of the framework.

This patch refurbishes the granting mechanism such that keystore can
choose a name for the grant. It uses the original symbolic key name as
prefix and appends _KEYSTOREGRANT_<grant_no> where the grant_no is
chosen as first free slot starting from 0. Each uid has its own grant_no
space.

This changes the grant call such that it now returns a string, which is
the alias name of the newly created grant. The string is empty if the
grant operation failed.

As before apps can still mask granted keys by importing a key with the
exact same name including the added suffix. But everybody deserves the
right to shoot themselves in the foot if they really want to.

Bug: 37264540
Bug: 62237038
Test: run cts-dev --module CtsDevicePolicyManagerTestCases --test
          com.android.cts.devicepolicy.DeviceOwnerTest#testKeyManagement
	  because it grants a key
Merged-In: I047512ba345c25e6e691e78f7a37fc3f97b95d32
Change-Id: I047512ba345c25e6e691e78f7a37fc3f97b95d32