Prepare dir structure for multi-arch prebuilts.

The change is currently a no-op. The prebuilts directories contain
copies of the currently available prebuilts (x86). A new drop of
prebuilts will follow in a separate CL.

The Android.bp was modified with the following command:
sed -i -E 'sA  src: "(.+)"A  arch: {\n    arm: {\n      src: "prebuilts/arm/\1",\n    },\n    arm64: {\n      src: "prebuilts/arm/\1",\n    },\n    x86: {\n      src: "prebuilts/x86/\1",\n    },\n    x86_64: {\n      src: "prebuilts/x86/\1",\n    },\n  }Ag' Android.bp

Bug: 148489495
Test: mm
Change-Id: Ie6926134a708a92a3ad8bfd53d8bc993358bb8c5
35 files changed