Switch usages of legacy-test to android.test.base

The legacy-test library has been deprecated in favour of the
android.test.base library which will be exposed to developers in
similar manner to android.test.runner and android.test.mock.

Bug: 30188076
Test: make checkbuild && make droid && flash
Change-Id: I36a9ccf8a1b4b7b75ce1ca4893daa3db6e660b30