Refactor BpfNetMaps and getChainEnabled

Address comments from aosp/2117045 and aosp/2131752
Rename USE_NETD to PRE_T
Rename getChainEnabled to isChainEnabled
Remove unnecessary parentheses
Fix comment

Bug: 217624062
Test: atest BpfNetMapsTest
Change-Id: Iaff8c9fc5f74de3fe41a7fb010355b1742fbce90
4 files changed