No need to 'addChild' when start 'FLAG_ACTIVITY_CLEAR_TOP' Activity.

Android R refactor Activity launchMode:'FLAG_ACTIVITY_CLEAR_TOP',
'singleTop','singleInstance'. After Android R restructure, the
'addChild' method leads to Activity leaks. No need to 'addChild'
in the current Android version anymore.

Test: build pass, feature pass.
TestCase: Start a Activity twice with flag 'FLAG_ACTIVITY_CLEAR_TOP'.
Reproductivity: always.

Signed-off-by: yangyankai1 <yangyankai1@xiaomi.com>
Change-Id: Ia4d5b875a6592b6448af7500626b5a273bd96dc3