Add a flag to allow unbundled builds to build SDKs from source

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