122452571: Fix path to android.jar in prebuilts

The --android-jar-pattern flag passed to metalava is used to
find all the prebuilts for previous API levels such that
it can compute the exact API level for each class and member.

At some point the prebuilts must have been moved without this
code getting updated.

This fixes
Bug: 122452571: Can't build app with Q prebuilt SDK with minSdkVersion=21
Test: make sdk; inspect api-versions.xml

Change-Id: I25f6203725391dc5404cb8b6049479c9c3da1699