Allow the skeleton of "jmma" in DateFormat.getBestDateTimePattern()

'a' and 'j' specify the intent to use 12-hour format,
and the locale default 12-hour format, and they shouldn't be used
in the same skeleton, because they contradict to each other.

A skeleton, e.g. "jmma", is mapped into "ahmma" internally
by #mapSkeletonMetacharacters, and thus the internal skeleton
has duplicated fields "a".

This is a regression when we moved the implementation of
android.text.format.DateFormat#getBestPattern() from
udatpg_getBestPattern in ICU4C to ICU4j DateTimePatternGenerator
in the CL https://r.android.com/1355735.
The expected pattern generated from skeleton of "jmma" is
"h:mm a".

Bug: 170233598
Test: DateFormatTest
Change-Id: I13929f4b5bc66aaa90e0cef666cf52d7c32122ac