Add tempPathForRestat to improve consistency

Previously, there were two approaches used to construct the temporary
path needed by the commitChangeForRestat method and neither was
suitable for general use. One was:
    android.PathForOutput(ctx, outputPath.Rel()+".tmp")
The other was:
    combinedAidl.ReplaceExtension(ctx, "aidl.tmp")

The first approach would not work if the supplied path had been created
by PathForModuleOut() or similar as it would drop the module directory.
That could lead to the same path being used for multiple rules.

The second approach would not work for files with a different
extension.

The tempPathForRestat combines the two approaches so it can be used
generally with any WritablePath.

Bug: 179354495
Test: verified that the ninja rules that used these files were not
      changed by these changes.
Change-Id: I4439dea0a823512c281eeb1366522fb49dceb4e3