Adding a test to simulate `OPERATION_BUSY` error from keystore.

The test tries to create a race condition: It creates an operation
and starts two threads, each trying to use the operation repeatedly
until it gets  `OPERATION_BUSY`  or finishes successfully.

Bug: 194359114
Test: atest keystore2_client_test
Change-Id: Ib3549d6bcad4e24bf621960f0f399481bf2da7c7
1 file changed