Convert NotificationContentInflater to singleton

Convert NotificationContentInflater into a singleton since it has
almost no state, making it easier to inject and re-use.

Move the only per-row state it has, cached remote views, to a separate
class NotifRemoteViewCacheImpl that manages the lifetime of the cached
remote views based off the notification's lifecycle. This has the added
benefit of being re-usable for content recycling later on.

Bug: 145749521
Test: atest SystemUITests
Test: atest NotificationContentInflaterTest w/o @Suppress locally
Test: smoke test
Change-Id: Iedda71d7e5edea9e30580e99eb5b6ec545bb708d