Use FLAG_IMMUTABLE for InboundSmsHandler

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: Idc684438d2cd622adee9a1c4d1801e308ee5ead7
Merged-In: Idc684438d2cd622adee9a1c4d1801e308ee5ead7
1 file changed