Fix failure of libvbmeta_test

The test uses 'external/avb/test/data/testkey_rsa2048.pem' from the
source tree, which is not available when running the test. Copy
the test key with the test case in Android.bp to fix the issue.

Bug: 177906739
Test: atest libvbmeta_test

Change-Id: I528dcdc5b48ed4af36ddd360380eb39631ff4317