Introduce InputMethodSubtypeSafeList

IMM#getEnabledInputMethodSubtypeList() can return a large list of
subtypes, which may cause a TransactionTooLargeException.

This patch introduces InputMethodSubtypeSafeList to wrap the list as a
byte array, avoiding the exception. This mirrors the existing
InputMethodInfoSafeList pattern introduced in [1].

Additionally, this change extracts the common marshalling logic from
InputMethodInfoSafeList into a new AbstractSafeList and refactors both
SafeList classes to extend it.

[1] I0a7667070fcdf17d34b248a5988c38064588718a

Bug: 449416164
Bug: 449181366
Bug: 449393786
Bug: 449227003
Test: CtsInputMethodTestCases:{InputMethodRegistrationTest,InputMethodInfoTest}
Test: InputMethodCoreTests
Flag: EXEMPT BUGFIX
(cherry picked from commit 1d68a1099be2b99e8410dad01822851287994682)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:945058179409359060bc68e2915a33416fa42ee8
Merged-In: Ied64a9f018fd3e79cfc51ccd82d361b43e5f29dc
Change-Id: Ied64a9f018fd3e79cfc51ccd82d361b43e5f29dc