Support VNDK extensions

This commit adds `extends: "name"` property and provides basic support
to VNDK extensions.  This is the simplest example:

```
cc_library {
    name: "libvndk",
    vendor_available: true,
    vndk {
        enabled: true,
    },
}

cc_library {
    name: "libvndk_ext",
    vendor: true,
    vndk: {
        enabled: true,
        extends: "libvndk",
    },
}
```

A vndk extension library must extend an existing vndk library which has
`vendor_available: true`.  These two libraries must have the same
`support_system_process` property.

VNDK-ext libraries are installed to `/vendor/lib[64]/vndk` and
VNDK-SP-ext libraries are installed to `/vendor/lib[64]/vndk-sp` by
default.

If there is a matching abi-dumps in `prebuilts/abi-dumps`,
`header-abi-diff` will be invoked to check for ABI breakages.

Bug: 38340960

Test: lunch aosp_walleye-userdebug && make -j8   # runs unit tests

Test: lunch aosp_arm-userdebug && make -j8  # build a target w/o VNDK

Test: Create a lsdump for a vndk lib, add an exported API to vndk lib,
and build fails as expected.

Test: Create a lsdump for a vndk lib, create an vndk extension lib with
extra API, and build succeeds as expected.

Test: Create libutils_ext, add an extra function to libutils_ext, and
call it from a HIDL service.

Change-Id: Iba90e08848ee99814405457f047321e6b52b2df0