Create soong_injection files in ReadWrite mode

The synthetic soong_injection workspace is shared between api_bp2build
(used in API export) and bp2build (used in API domain analysis which
runs mixed builds by default). The former creates them in ReadOnly mode,
but the latter creates them in ReadWrite mode (to allow users to
edit/experiment). To prevent errors in the end-to-end builds in
multittree, make api_bp2build create these files in ReadWrite mode as
well.

Test: mulittree_build locally
Change-Id: I8766d88951a3b625739e4dc12d468450ad7322ea