Introduce TARGET_BUILD_UNBUNDLED.

Setting this flag enables unbundled building, i.e. without support for
building the system image and other platform targets. This
functionality was previously enabled by TARGET_BUILD_APPS, and setting
that still implies TARGET_BUILD_UNBUNDLED.

This helps unbundled builds that aren't apps, e.g. ART runtest builds.
Specifically, with the topic of the child CL
https://r.android.com/1324517 TARGET_BUILD_UNBUNDLED does not imply
disabling dexpreopting, unlike TARGET_BUILD_APPS.

TODO: There may still be app-specific conditions that are incorrectly
controlled by TARGET_BUILD_UNBUNDLED, in particular on the Soong side
through config.UnbundledBuild().

Test: Flash & boot
Test: TH, in particular builds green on ub-launcher3-master
Bug: 157549171
Change-Id: Ic09fc879117ee06cab5444edfc280ed2b52d2870