Verify java.text.SimpleDateFormat against 8u121-b13 (part 1)

- Added missing documentation for symbols L and M.

- Documented removed code:
  * Field forceStandaloneForm
  * Unused local variables inside compile() method
  * Unused private method matchDSTString()
  * Removed redundant change markers

- Integrated differences with upstream not present in code:
  * actualStart variable in subParse()
  * Split applyPattern() and applyPatternImpl()
  * Inlined isGregorianCalendar() and getCalendarName()
  * Fix wrong space counting for yy format strings

- Documented Android changes:
  * Extracted code to separate methods from switch cases in subFormat(),
    including formatMonth() and formatWeekday() methods for 'M' and 'E'
    symbols
  * Reimplement 'z' symbol handling using ICU
  * Reimplement 'Z' symbol using TimeZone class
 *  Be more tolerant of colon. http://b/26426526

- Add two test cases covering M modifier behavior for both Gregorian and
  non-Gregorian calendars.

Bug: 66411240
Test: cts-tradefed run cts -m CtsLibcoreTestCases --abi arm64-v8a
Change-Id: I3e862deb38e3d2b3ba0ac0e22e3753117a311007