Fix two incorrect test conditions in `libartbase/base/flags_test.cc`.

Fix conditions in the following `art_libartbase_tests` tests, which
used to be inverted:
- `FlagsTestsWithCmdLine.FlagsTestsGetValueServerSetting`
- `FlagsTestsCmdLineOnly.CmdlineOnlyFlags`

Test: atest art_standalone_libartbase_tests
Test: Run `art_libartbase_tests` with original chroot-based harness
Bug: 190150217
Change-Id: Id3d1aa74fd435c82b6806af5a0b9b5eab8371dfb