Cleanup custom IActivityManager.cpp binder code

Make sure to check the return value from transact
in all places, and make sure not to read from the
reply Parcel for oneway transactions.

Test: atest
Flag: EXEMPT small bugfix
Ignore-AOSP-First: Part of larger internal work
Change-Id: I85653d9de84bbe7ba40d9675bb0e02263efe89a9