Support building WITHOUT_CHECK_API=true

When WITHOUT_CHECK_API=true the check api process is not run which
means that the current.txt and removed.txt files are not generated by
the build. In that case this change causes the droidstubs module to
fallback to exporting the source files instead.

An additional removedApiFilePath property (to match apiFilePath) was
added to hold the exported path as removedApiFile has to be a
WritablePath and source paths are not writable.

Bug: 169034951
Test: m WITHOUT_CHECK_API=true nothing
      fails without this fix, passes with it.
Change-Id: If6b09dd67c8533368b71405143f524d66350703f