Handle SubSettings instance leaks in Index.java

Index's constructor will hold context of caller.
context.getApplicationContext() instead of context to prevent activity
leaks.

Bug: https://code.google.com/p/android/issues/detail?id=223322



Test: manual-start a new activity call Index constructor ,then pressed
back.

Change-Id: Ic12af62f9b718d328610a57985f64ca4629bab99
Signed-off-by: gaochong <gaochong@xiaomi.com>
(cherry picked from commit 8eaaba9b)