Add throwIfParentInstance check to setMandatoryBackupTransport.

Throw SecurityException from setMandatoryBackupTransport if
in a parent instance.

BUG: 72332943
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases --test
com.android.cts.devicepolicy.ManagedProfileTest#testParentProfileApiDisabled

Change-Id: Iff2367256ce372641fd5569ad27b32f0894597ce