Touch up manifest if there's no source code.

The new package manager behavior requires packages without source code
to have an application element with hasCode attribute set to false in
their manifest. With this change, Soong can now automatically insert one
for codeless apps.

Test: app_test.go, manifest_fixer_test.py
Fixes: 124375490
Change-Id: Ied89a8d07c63805ab910859a4f7c45fc1c60bb73