Cache Unicode extension subtag key map in Zygote

Save 1 JNI transition by moving the cache loading from
LocaleNative.cacheUnicodeExtensionSubtagsKeyMap()
to ZygoteInit.preloadTextResources()

Bug: 355467371
Test: perfetto trace shows the event
Change-Id: Idaa009e6b742479dce4dcec4a79b7a14e5b9a4a6