Add property for incremental nsjail genrules

Normally genrule sandboxes run the build command in a clean state.
Setting keep_gendir as true, along with use_nsjail, will keep $(genDir)
so the genrule can be incrementally built.

Bug: 381459587
Test: build with and without the flag
Change-Id: I07bbea965f7b644ee8c8d2ead5b6abdd1f0c9aa6