Update in DPM delegation documentation.

Add note on DPM#setDelegatedScopes documentation regarding the
broadcast sent to the delegate package to notify its new scopes; and
change the admin ComponentName annotation to @Nullable in
DPM#getDelegatedScopes.

Bug: 33099995
Test: cts-tradefed run cts-dev --module CtsDevicePolicyManagerTestCases --test com.android.cts.devicepolicy.MixedDeviceOwnerTest#testDelegation
Change-Id: I28fe3a631c05a9e6b8dae766ce6c42881f2e3a00