Mainline modules are tightly coupled to the platform, and should
build against the current SDK from source and not prebuilts. Add
a flag UNBUNDLED_BUILD_SDKS_FROM_SOURCE to specify that a
TARGET_BUILD_APPS build should build the current SDK instead of
using the prebuilts.
Bug: 121194841
Bug: 121231426
Test: no change to out/build-aosp_sailfish.ninja
Test: forrest unbundled build
Test: forrest master apps build
Test: forrest mainline modules build
Change-Id: I45a40a335483dae2fe192721df9b31bdbab97ee5