WAPPushManager: Add an exported flag in manifest

With b/150232615, we will need an explicit value set for the exported
flag when intent filters are present, as the default behavior is
changing for S+. This change adds the value reflecting the previous
default to the manifest.

Bug: 150232615
Test: TH
Change-Id: I2ad8dd831d6a2a9cacd5ca3671358b961c781a7b