[BUG]: Back from player opens a duplicate channel page (extra Back to leave a channel)

[CodeSculptor]

Summary

When a video is opened from a channel page and the user presses Back to leave the player, a
second, fresh channel activity is launched on top of the existing one. The user then has to
press Back twice to leave the channel (the first Back dismisses the duplicate channel page, which
looks identical to the one beneath it).

Repro (on v0.4.2-beta.7+st31.94, device R3GL3069JMZ):

1. Subscriptions → Channels → tap a channel (Channel page opens)
2. Tap a video in that channel (player opens, plays)
3. Back → returns to the channel page ✓
4. Back → shows the same channel page again (the duplicate) ✗ — expected to return to the Channels list
5. Back → returns to the Channels list

Root cause

MobilePlaybackActivity.onBackPressed() (the #23 fix) calls finishReally(). The inherited
LeanbackActivity.finishReally() calls getViewManager().startParentView(this) before finishing
(smarttubetv/src/main/.../tv/ui/common/LeanbackActivity.java:131). startParentView
(common/.../app/views/ViewManager.java:132) launches the parent (channel) via a plain
new Intent(activity, MobileChannelActivity). MobileChannelActivity is launchMode=singleTop,
which only reuses an instance that is already at the top of the task — but the original channel
sits beneath the player (in a separate task), so a new MobileChannelActivity is created.

Activity trace confirming the second channel launch on Back:

START ... MobileChannelActivity   (tap channel)
START ... MobilePlaybackActivity  (tap video)
START ... MobileChannelActivity   (~Back from player — the duplicate)

Every finish() / finishReally() branch except the bare super.finish() calls
startParentView, so there is no trivial stmobile-only one-liner.

Notes

• Pre-existing, not introduced by [BUG]: Player <-> channel Back loop with lingering audio #23. In beta.6 the old finish() path also called
  startParentView (duplicate channel) and kept the engine alive (audio loop). [BUG]: Player <-> channel Back loop with lingering audio #23 fixed the
  audio loop; this duplicate-channel remainder became visible once Back actually leaves.
• Severity: minor (one extra Back; no crash, no data loss, no lingering audio).

Fix options (to scope)

1. Shared (clean, but mergeability cost): make ViewManager.startParentView reuse an existing
   parent instance (e.g. moveTaskToFront / FLAG_ACTIVITY_REORDER_TO_FRONT/CLEAR_TOP) instead
   of always starting a new one. Touches common → weigh upstream-merge impact.
2. stmobile-only workaround: have MobilePlaybackActivity.onBackPressed stop playback and
   finishAndRemoveTask() directly (bypassing startParentView) so the player's task is removed
   and the system returns to the channel's task; or give MobileChannelActivity a singleTask
   launch mode. Both risk regressing [BUG]: Player <-> channel Back loop with lingering audio #23's PIP / background-audio paths and need device testing.

Deferred from the beta.7 release (shipped as-is; beta.7 is strictly better than beta.6).