Update droidstubs to use the SdkExt info file

metalava now accepts the --sdk-extensions-info flag. The argument is a
file containing metadata regarding extension SDKs; the SdkExtensions
module defines such a file. Update the droidstub targets to point to
this file.

Bug: 228828986
Test: m sdk dist
Change-Id: I3a5d92c640d9b22add416a20dbec3c8e352ba963