Rework prependAS()

Rework prependAS()

We cannot modify lists in-place, so make a proper copy and create
a new list. Fixes the following splat:

java.lang.UnsupportedOperationException: null
	at com.google.common.collect.ImmutableList.remove(ImmutableList.java:534) ~[36:com.google.guava:27.1.0.jre]
	at org.opendaylight.protocol.bgp.openconfig.routing.policy.statement.actions.AbstractPrependAsPath.prependAS(AbstractPrependAsPath.java:38) ~[232:org.opendaylight.bgpcep.bgp-openconfig-rp-statement:0.13.0.SNAPSHOT]
	at org.opendaylight.protocol.bgp.openconfig.routing.policy.statement.actions.LocalAsPathPrependHandler.applyImportAction(LocalAsPathPrependHandler.java:37) ~[232:org.opendaylight.bgpcep.bgp-openconfig-rp-statement:0.13.0.SNAPSHOT]
	at org.opendaylight.protocol.bgp.openconfig.routing.policy.statement.actions.LocalAsPathPrependHandler.applyImportAction(LocalAsPathPrependHandler.java:18) ~[232:org.opendaylight.bgpcep.bgp-openconfig-rp-statement:0.13.0.SNAPSHOT]
	at org.opendaylight.protocol.bgp.openconfig.routing.policy.spi.registry.ActionsRegistryImpl.applyImportAction(ActionsRegistryImpl.java:314) ~[231:org.opendaylight.bgpcep.bgp-openconfig-rp-spi:0.13.0.SNAPSHOT]
	at org.opendaylight.protocol.bgp.openconfig.routing.policy.spi.registry.StatementRegistry.applyImportStatement(StatementRegistry.java:82) ~[231:org.opendaylight.bgpcep.bgp-openconfig-rp-spi:0.13.0.SNAPSHOT]

Change-Id: I6f604e8abdb223fe09e44ac92a752bb6e410ed7f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>