Add concept of implicitly exposed components

Implicitly exposed components are those that can still be accessed
via generic startActivity() calls. However, they cannot be started
by specifying a package or component. Additionally, package info
can't be obtained if the package only has implicitly shared
components.

Change-Id: I404a4dff87559cfeee6ad78f7dcc7f5d849d6869
Fixes: b_37343345
Test: cts-tradefed run commandAndExit cts-dev -m CtsAppSecurityHostTestCases -t android.appsecurity.cts.EphemeralTest