Select only preinstalled Spell Checker Services

When we are setting a new spell checker as the default one in
Secure.Settings, TSMS#findAvailSpellCheckerLocked can pick up
any available spell checker service. This violates the principle
that user should be warned whenever we are setting an untrusted
spell checker service as the default service, since the warning
dialog is never shown.

Fixes: 64764051
Test: Manually as follows:
0. Make sure AOSP keyboard is pre-installed.
1. adb shell settings put --user 0 secure selected_spell_checker com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService
2. tapas SampleSpellCheckerService
3. make -j
4. adb install --user 0  -r out/target/product/generic/system/app/SampleSpellCheckerService/SampleSpellCheckerService.apk
5. adb shell pm disable com.android.inputmethod.latin
6. adb shell settings get --user 0 secure selected_spell_checker
    -> com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService
7. adb reboot
8. adb shell settings get --user 0 secure selected_spell_checker
    -> com.android.inputmethod.latin/.spellcheck.AndroidSpellCheckerService
Change-Id: I298ffbcfa5e32f43753f54fbebc40a414a5c0f9e