Properly initialize android_system_image modules

android_system_image just passed its embedded filesystem module to
InitAndroidModule (transitively via initFilesystemModule). This meant
that ModuleBase.module was pointing to the filesystem module, not
the system image module. This was a problem for the new "changeable
configuration" feature I'm working on, but you can also more readily
see the issue if you add a GenerateAndroidBuildActions function to
android_system_image: it won't be called.

Bug: 361816274
Test: Presubmits
Change-Id: I41978037b81910309a000dc1bba327e513ba5dce