Require explicit `android:exported` in manifests

Starting S+, if an intent filter is defined, we are requiring that the
android:exported flag also be explicitly defined in the manifest. With
this change apps targeting S+  that do not meet this requirement will
fail to install.

There is no change in the default behavior of the `android:exported`
flag when there is no intent filter defined (i.e. the flag defaults to
false in these cases).

Bug: 150232615
Test: Flashed the build on a device and tested with test apps
Change-Id: I36efeebf28f21a4d4277f40a17e38d2183514037