Fix difference in default `cc_test.isolated` in soong and bp2build

Soong defaults to false, while bp2build defaults to true. This CL fixes
bp2build's default to false to match Soong.

To prevent future differences, bp2build now uses the `isolated()`
function used by Soong. The type of the context object has been changed
to EarlyModuleContext so that the ctx object does not depend on
mutations that might not run in bp2build.

Test: TH
Bug: 244432609
Change-Id: I15346107896312961e8d12270cf2f9a2a48827e1