Add support for ICU data pinning in the Zygote

Upstream ICU caches use SoftReferences. On Android this means
that useful cached data initialized in the Zygote are "lost" when
the Zygote GCs and cannot be shared with apps. This change makes use
of an Android patch to ICU to ensure References created during
Zygote initialization are "strong". i.e. they are never collected.
This prevents them being GCd and ensures they can be shared between
applications.

After switching ICU to use strong references, this change
also creates DecimalFormatSymbols objects for common ULocales
(ROOT, US and the user's default, if different). DecimalFormatSymbols
makes use of an ICU Reference cache and this alone has been shown to
improve the construction time of java.text.DecimalFormat by 1-1.5
milliseconds on a Seed device. This saving applies the first time one
is created in each app for each locale, and again if SoftReferences
have been cleared.

The cost to the heap size of the Zygote has been measured at ~107k.
This value will change as more caches are switched to use the new
CacheValue class.

Formatting is typically performed on the UI thread and the intention
of this change is to reduce app start up time and jank in apps like
the Dialer which do a lot of formatting when scrolling lists. The
change may also enable more virtual memory page-sharing between
apps, though this is not the specific goal.

Bug: 28326526
(cherry picked from commit 41c9dc3b)

Change-Id: I48e4d57ecbb207b9a5e17b6caf5e7b282e4a40e3