Clean up BLOCK_ACTIVITY_STARTS_AFTER_HOME_FLAG.

This flag was only introduced to allow teamfood testing of the change
to use background activity launch blocking instead of delaying app
launches after app switching protection is engaged. The flag has been
on everywhere for 2 weeks (ag/176961146) without problems and is no
longer needed.

This CL went in three stages:

- Change getBalAppSwitchesProtectionEnabled() in
  ActivityTaskManagerService to always return true.

- Inline the function (and selected others) into call sites and then
  eliminate the resulting dead code - done as a serious of automatic
  refactorings to minimise the risk of human error.

- Delete unit tests for code that no longer exists.

Apart from deleting the use of the flag I believe this CL is a pure
refactoring with no new behavior changes.

The end result is we eliminate quite a lot of code - especially
PendingActivityLaunch and all uses of it.

Test: atest BackgroundActivityLauncherTest
Test: atest ActivityStarterTests (both of them)
Test: manual
Bug: 176961146
Bug: 159433730
Change-Id: I01625843f67b00e5901833cff99aec16c1287a0c