Change type of TARGET_BUILD_APPS from bool to list

TARGET_BUILD_APPS is supposed to be a list of apps to
build, so having it be a bool was incorrect. Change it
to properly reflect what it is, because the starlark
product configuration relies on soong's interpretation
of variables types.

Bug: 201700692
Test: Presubmits
Change-Id: I00109b57abbcda86077f0465f982f54b4a77a7f0