DevicePolicyManager: clean up calls to getCallerIdentity()

CallerIdentity objects should be constructed at IPC boundary and
passed along the call chain.

Bug: 431159719
Test: atest FrameworksServicesTests:com.android.server.devicepolicy
Flag: EXEMPT refactor
Change-Id: I8a1d5edf508904c7c99856040e872b0abd6acbe3