Parse share-targets defined by non-main activities

Some system apps do not have any launcher icons but want to publish
sharing shortcuts. As a fix, all activities are parsed for their XML
resource, which implies some of these shortcut may not hold an
activity. This CL removes that assumption from launcher side.

Bug: 136353429
Test: Manual test
Change-Id: I2994da17fe078b2fab61a614c1326fddd5fd8571