Do not hold g_namespace_mutex during library loads.

Significantly shortens the locked durations, and makes it easier to
ascertain there's no reentrancy through the load code paths.

Test: atest libnativeloader_e2e_tests libnativeloader_test \
            art_libnativeloader_cts_test libnativeloader_lazy_test
  with and without CHECK condition inverted
Bug: 326505705
Bug: 326610154
Change-Id: Ib2290714fd102ae22f960e9f631c1623236f4d27