Add a PendingIntent flag to mark unaudited PI mutability

This change adds a FLAG_MUTABLE_UNAUDITED that is equal in value to
FLAG_MUTABLE but is intended to be used with PendingIntents that have
not yet been audited to be intentionally mutable/immutable. This will
help us update the tree to work with the enforced explicit mutability flag
requirement (go/immutable-pendingintents has more context).

This flag is marked as deprecated and we hope to remove it once all PIs
have been audited and use either FLAG_MUTABLE or FLAG_IMMUTABLE.
FLAG_MUTABLE_UNAUDITED should not be relied on long term, and any usage
is intended to be short term.

Bug: 160794467
Test: TH
Change-Id: I6a4f65d5364a611620b88e92c208aba7faaa13b8