Fix some checkstyle nits in DhcpPacketTest.

Use the Generated code suppression comment filter instead of the
IndentationCheck suppression comment filter because the latter
complains that '+' should be on a new line.

Bug: 120584519
Bug: 127423755
Test: atest NetworkStackTests
Test: atest FrameworksNetTests
Change-Id: I35b8c943b95a4301c25b7f6cfb02f03a8c1bc34e
Merged-In: I60071bc029d25485bf204cfd3a8cebd538ca12b6
(cherry picked from commit 65f6ae27)