Statically link libbase for mainline module tests

b/310046696 introduced a breaking ABI change in libbase, and
the following unit tests fail to launch on previous
platforms because of missing symbols:
  - libnetworkstats_test
  - netd_updatable_unit_test

Bug: 316464217
Test: atest libnetworkstats_test
Test: atest netd_updatable_unit_test
Change-Id: Icef58de0b46c6042a9459d3d4c45ea9ce2baefed