Add proper padding to PerfTestActivity

Since Android V, apps targeting SDK 35+ will be forced to go
edge-to-edge. This CL adds proper padding to the content view of
PerfTestActivity, so the EditText within it won't be obscured by system
bars. Otherwise, there would be unexpected scrolling while showing or
hiding IME.

This CL also removes the action bar from the activity, so that the
content won't be obscured by it.

Fix: 331025926
Bug: 309578419
Flag: com.android.window.flags.enforce_edge_to_edge
Test: ImePerfTest
Change-Id: I212ef4862b4249e288267fb8fb4448b9100107c1