Unhide getLockTaskPackages

getLockTaskPackages is currently hidden, and can only be
called by a device or profile owner, which doesn't make
much sense. Unhidding it to be consistent with the rest
of the DevicePolicyManager APIs that have a getter for
each setter.

Bug: 34614754
Test: cts-tradefed run cts-dev --module DevicePolicyManager --test com.android.cts.devicepolicy.DeviceOwnerTest#testLockTask_affiliatedSecondaryUser
Test: cts-tradefed run cts-dev --module DevicePolicyManager --test com.android.cts.devicepolicy.DeviceOwnerTest#testLockTask_unaffiliatedUser
Test: Test:  cts-tradefed run cts-dev --module DevicePolicyManager --test com.android.cts.devicepolicy.DeviceOwnerTest#testLockTask_deviceOwnerUser
Change-Id: I6e03c2f47c0f9e7a635e798a1bf7f131a8e37c65