Replace IMMS#mAdditionalSubtypeMap with AdditionalSubtypeMapRepository

With this commit

  InputMethodManagerService#mAdditionalSubtypeMap

will be superseded by a newly introduced

  AdditionalSubtypeMapRepository

so that all users' additional subtypes can be tracked equally rather
than treating current user's one specially. A notable improvement is
that this change allows us to save unnecessary file loading of
additional subtype persistent files, which may help us mitigate the
potential regression discussed in Bug 327861441.

There must be no observable behavior change in this commit.

Bug: 309837937
Test: atest CtsInputMethodTestCases:InputMethodSubtypeEndToEndTest
Test: atest CtsInputMethodInstallTestCases:AdditionalSubtypeLifecycleTest
Test: atest FrameworksInputMethodSystemServerTests
Change-Id: Ic003648cfc9950f190a6b3fec7fd8fb363bfe081