Update SOONG_PARTIAL_COMPILE logic

Separate PARTIAL_COMPILE changes that affect analysis from actually
using it.

- SOONG_PARTIAL_COMPILE affects how we generate the Ninja file.  Changes
  to this cause reanalysis.

- SOONG_USE_PARTIAL_COMPILE is used in the rules to determine if we use
  the PARTIAL_COMPILE implementation, or the legacy rules.  This means
  that the developer can switch between full compiles and partial
  compiles at will without waiting for analysis to happen each time they
  change it.

Bug: b/365536323
Test: manual
Change-Id: Icb06687d17f63edb22bf2457a659b452537dadba