Propagate TARGET_BUILD_APPS to Soong.

Used to select SDK variants of native libraries when building unbundled
apps only. This allows unbundled modules to access the platform
variants (and typically their versioned APEX stubs).

Test: m (on AOSP master)
Test: TH (including ub-launcher3-master)
Bug: 157549171
Change-Id: I74f0bcebe8b66b81b2c80e9d448f5d24dc9ab991