Fix is installed check condition in DevicePolicyManagerService

DPMS#setDelegatedScopes generally enforces the delegate is installed in
the device, but this check should be skipped on DELEGATION_CERT_INSTALL
scopes on pre-N. Additionally the check is also skipped when clearing up
delegations on pre-N. The check was extracted to a separate function for
clarity.

Bug: 35234284
Test: cts-tradefed run cts-dev --module CtsDevicePolicyManagerTestCases --test com.android.cts.devicepolicy.ProfileOwnerTestApi23#testDelegatedCertInstaller
Change-Id: Ib723b58243f901af907e368017b1ae0bb101360d