Refactor GetTargetProperties().

This is so that its implementation is more amenable to implementing
`target: { android_arm: { ... } }` and the like.

In addition, add a Boolean return value to getChildPropertyStruct() and
use it at every call site so that we only merge property structs when it
makes sense.

Bug: 187530594
Test: Presubmits.
Change-Id: I1b441c14b3d90a12b84dc89f82dd053978e89c7e