Pass --sdk-extensions-{root,info} to metalava

Teach soong to pass metalava the two new command line arguments
--sdk-extensions-root <dir> and --sdk-extensions-info <file> when
generating the API levels file (--generate-api-levels api-versions.xml).

The directory hierarcy in a droiddoc_exported_dir module has special
meaning, e.g. extensions/1/public/*-stubs.jar are the mainline module
stubs containing the public API for SdkExt version 1. Update the logic
where the directories are scanned for android.jar files to locate the
SDK extension jars (extensions/<int>/public/*-stubs.jar).

Also introduce a new field on droidstubs properties
(Extensions_info_file) to set the value of --sdk-extensions-info.

Note: if Extensions_info_file is not set, neither
--sdk-extensions-root or --sdk-extensions-info will be passed to
metalava.

Bug: 228828986
Test: go test -run TestDroidstubsWithSdkExtensions ./java
Test: m sdk dist && grep -e from= -e module= out/dist/data/api-versions.xml # needs APIs to be listed in the info file
Change-Id: I682e34d328fc93d3eded8565ffee40961307901a