system/gd/packet: Migrate from ASSERT/ASSERT_LOG to std::assert

log::assert_that cannot be used in this context, because
libbluetooth_hci_pdl is linked in //vendor, whereas
libbluetooth_log will NOT be made visible to //vendor

Test: m com.android.btservices
Bug: 305066880
Flag: EXEMPT, log change
Change-Id: I469c1cda0787c00eb04b1ac92228202d56fa0ace