Permission tests: Press back only when necessary

These tests blindly press back multiple times to dismiss dialogs or
overlays.
On TV, when pressing back while on the launcher, the TV enters dream.
To avoid tests entering dream, remove unnecessary back presses.

Bug: 193031221
Bug: 192107479
Bug: 194270247
Test: atest CtsPermission3TestCases CtsPermission4TestCases (on adt3)
Change-Id: I8a1887d4944751009bb33123716641fab6c9252d
2 files changed