[SettingsProvider] fix byte count

Memory usage per package was limited by "bytes" but counted by
"character length". Every character is 2 bytes in Java. This CL fixes
the counting and also doubles the bytes limit so that it doesn't cause
any behavior changes.

BUG: 253499273
Test: atest com.android.providers.settings.SettingsStateTest
Change-Id: I7a8be794582c153adf43ec735d2563695ea1533d