media: reduce uncertainty in ResourceManagerTest

ResourceManagerTest tests codec relaiming by launching two activities,
with Activity1 creates max number of codecs then moves itself to back,
then Activity2 tries to reclaim from Activity1. This reclaim's success
relies on the fact Activity2's process is running at higher priority
then Acitivity1 when the reclaim request is issued.

Currently, Activity2 tries to do reclaim from onCreate(). However,
the activity is not made visible until onStart() and is not top-running
until onResume().

Moving the reclaim attempt from onCreate() to onResume() to rule out
any uncertainty that could be associated with the visiblity change.

bug: 145780610
test: atest android.media.cts.ResourceManagerTest
Change-Id: I6743327c9499a976fee4a725e94a38c18e75df0e