Add PRODUCT_ADB_KEYS

This easily allow products to add custom adb keys for debuggable builds.
To use, provide a public key created by `adb keygen` to
PRODUCT_ADB_KEYS.

This way automated test farms don't need manual intervention to
authenticate to the device over adb, but we don't disable security for
everyone else.

Add an inherit-product-if-exists hook to aosp_* targets so that our
build servers can add a key for our test farms.

Bug: 32891559
Test: lunch aosp_marlin-userdebug; m bootimage
Test: lunch aosp_marlin-user; m bootimage
Change-Id: I1720644d89ec5289fbe99f95ebcdfbb3f3b20e67