PFK: fix race between key set and key invalidate in TZ

When working with multiple files and multiple threads, the following
scenario can occur:
1. File Close -> Key cache removal -> context switch
2. Open new file -> occupy the entry cleaned in 1
   -> TZ_ES_SET_ICE_KEY -> context switch
3. Back to 1 -> TZ_ES_INVALIDATE_ICE_KEY
4. Back to 2 -> ICE uses the key that is already invalid
5. Crash due to PARTIALLY_SET_KEY_USED

To fix this, pfk must know when requests using some key are completed.
Only then key removal shall be allowed and until then key invalidation
must wait.
A new callback was added to let clients inform PFK when requests end.

Change-Id: Id7f8a3302fac9fafd1203d8d56ca13d59b45bbd5
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>