Use FLAG_IMMUTABLE for CarrierKeyDownloadManager

Require that the PendingIntent be immutable so that a malicious app is
 not able to hijack and mutate any of the details.

Test: Android builds
Bug: 155094269
Change-Id: I223047644021a647bce72f229998487bee0c007b
Merged-In: I223047644021a647bce72f229998487bee0c007b
(cherry picked from commit 57fc1fb9e63f545e05ed8c9e3dc2c31c1a591116)
1 file changed