Add change comments in DecimalFormat/NumberFormat.

Add Android-changed/-added/-removed comments where approrpiate in
java.text.NumberFormat and java.text.DecimalFormat. This verifies that
this file is consistent with OpenJDK 8u121-b31.

Additionally this commit contains a number of small non-documentation
changes:

 - renamed init to initPattern to indicate parallel to applyPattern
 - Changed the order of isParseIntegerOnly/setParseIntegerOnly to be
   consistent with other getters/setters and replace documentation with
   {@inheritDoc} as no additional information was given.
 - re-added "fastPathCheckNeeded = true" commented-out in many places to
   make comparison with upstream easier.
 - move currentSerialVersion = 4 to bottom of file to be consistent with
   upstream.
 - re-added explicit "symbols = null" initialization to align more
   closely with upstream.

Bug: 65669355
Test: CtsLibcoreTestCases
Change-Id: Ib0d7d8f857f88d4eee2cce4c159e4e28b4fdc2fe
2 files changed