[Bold text] Use a default bold font for null typefaces

When a TextView's typeface is null and bold is enabled, Typeface.DEFAULT
is used. Instead, this should be Typeface.DEFAULT_BOLD

Flag:
adb shell device_config put text com.android.text.flags.fix_null_typeface_bolding true

Bug: 314811487
Test: manual, CTS
Change-Id: I1be9d6c17b6736caa28e280872bf4b1dac4c7855