Remove the enable_isolated_storage Soong variable

Remove conditional compilation of the isolated storage pVM payload. We
will now always compile the necessary functionality for isolated
storage and will rely on the aconfig flag to indicate use.

Test: AppSearch CTS Tests
Bug: 402773218
Flag: com.android.appsearch.flags.enable_isolated_storage
Change-Id: Ib468dd8f1fa378d0f17ffa5ebd96bc97952031d3