Add a mutability flag to the PendingIntent in SearchView.java

It is now required to specify an explicit mutability flag when creating
PendingIntents. This change adds an explicit FLAG_MUTABLE to a
PendingIntent in SearchView.java.

Since the previous default behavior was mutable, and the flag I have
added is FLAG_MUTABLE, this change should be a no-op functionally.

Bug: 170770665
Test: TH
Change-Id: Ib6d4f95f64430d8b187560614b97bac961a129c5