Clarified CrossProfileApps API and documentation.

Changed CrossProfileApps#startAnyActivity to #startAcitivty.

Updated documentation for the usage of startActivity vs startMainActivity.
Also clarified that the INTERACT_ACROSS_PROFILES permission is a two-way
channel between personal and managed profiles.

Bug: 122886462
Test: n/a (documentation changes only)
Change-Id: I67772aef0cf24110432a2dbde0f87c91354fd4a0