Refactor logcat collection in ActivityManagerService

- Fixed inconsistent log truncation between GMS and ActivityManager by
  respecting Dropbox max file size
- Moved logcat fetching logic out of AMS, to a dedicated `LogcatFetcher` class.
- Replaced AMS logcat retrieval with `LogcatFetcher.appendLogcatLogs()`.
- Improved logcat collection by adding a timestamp and considering dropbox size to limit logs in dropbox error entries.

Flag: com.android.server.am.limit_logcat_collection
Bug: 354138103
Test: atest FrameworksServicesTests:LogcatFetcherTest && manual testing
Change-Id: I720202c31a38a20e4d7eaf30b5f80ecdfcb4bf0f